Troubleshooting / Mitsubishi FX PLC errors

Mitsubishi FX PLC Error Codes

The error codes of Mitsubishi MELSEC FX PLCs (FX3U, FX3G and FX3S, and the FX5U iQ-F): what each one means, whether the PLC stops, the likely causes, the first checks with the registers to read, and how to log it so it stops coming back.

How errors show

FX3 series (FX3U, FX3UC, FX3G, FX3GC, FX3S): ERROR (ERR) LED flashing means a parameter, syntax or circuit (ladder) error, and the PLC will not go into RUN. ERROR LED lit means a watchdog timer error or a PLC hardware error, and the PLC is forced to STOP. ERROR LED off while the machine still misbehaves points to an I/O configuration, communication or operation error, and the PLC keeps running.

BATT LED lit (FX3U/FX3UC), or ALM LED lit (FX3G/FX3GC with the optional battery), means the battery voltage is below the level set in D8006. When M8060, M8061 or M8064 to M8067 turns on, M8004 turns on and D8004 holds the lowest error flag number (for example 8066).

The code itself is in the matching register: D8060 I/O configuration, D8061 PLC hardware, D8062 PLC/PP communication, D8063 serial channel 1, D8438 serial channel 2, D8064 parameter, D8065 syntax, D8066 circuit, D8067 operation. 0000 in these registers means no error. The step number of a syntax, circuit or operation error is in D8069 (FX3U/FX3UC also D8314 and D8315).

Read it in GX Works2 under Diagnostics, PLC Diagnostics, or on the FX3U-7DM display under ErrorCheck. To clear: fix the cause, then switch STOP to RUN (this clears M8060, M8061 and M8064 to M8067), or cycle power. The M8068 operation error latch (and D8068) stays on until you reset it with the device monitor in the programming tool.

FX5 series (iQ-F): the CPU turns on SM0 and SM1 and stores the latest code in SD0 (up to 16 current codes in SD10 to SD25); operation errors also set SM8067 and SD8067. All FX5 codes are 4-digit hex with an H suffix. ERR LED on means an error or hardware error. ERR LED flashing means factory default (no program written), an error, a hardware error or a reset in progress.

P.RUN LED off means stopped or a stop error. BAT LED flashing means a battery error (FX5U/FX5UC). Stop errors put the CPU in STOP; continuation errors let it keep running. Clear continuation errors by fixing the cause and switching SM50 off and on, or from module diagnostics in GX Works3. Stop errors clear after you fix the cause and reset the CPU module or cycle power.

Safety first.
Cut off all phases of the power supply externally before installing, wiring, cleaning or retightening terminals, and never touch a terminal while the PLC is powered.

Build the emergency stop, protection and interlock circuits outside the PLC. When the PLC detects an error such as a watchdog timer error it turns all outputs off, but a failed relay, triac or transistor output can stay on or stay off, so external circuits must keep the machine safe in that case.

Read the manuals and make sure the machine is safe before you change the program, force devices or switch the PLC to RUN. Turn the power off before you plug or unplug extension cables, the battery or the memory cassette. (FX3U Hardware Edition JY997D16501 and FX5 Hardware manual SH-082452ENG, safety precautions.) Follow lockout/tagout and your site's safe-work procedures, and check the manual for your exact model before you act.

Quick lookup

Errors that stop the CPU stop the machine, so in OEE they are availability losses; continuation errors let it keep running until you act.

Type a code or a word (battery, watchdog, instruction, communication) to filter the list. Click a code for its causes and first checks.

84 codes

CodePLCName
6503FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: No setting value after OUT T or OUT C / Insufficient number of operands for an applied instruction
6501FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: Incorrect combination of instruction, device symbol and device number
6504FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: Same label number is used more than once / Same interrupt input or high-speed counter input is used more than once
6506FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: Invalid instruction
6505FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: Device number is out of allowable range
6502FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: No OUT T or OUT C before setting value
6507FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: Invalid label number [P]
6510FX3U, FX3UC, FX3G, FX3GC, FX3SSyntax error: MC nesting number error
6610FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: LD, LDI is continuously used 9 times or more
6611FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: More ANB/ORB instructions than LD/LDI instructions
6612FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: Less ANB/ORB instructions than LD/LDI instructions
6613FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: MPS is continuously used 12 times or more
6614FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No MPS instruction
6615FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No MPP instruction
6616FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No coil between MPS, MRD and MPP, or incorrect combination
6617FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: Instruction below is not connected to bus line: STL, RET, MCR, P, I, DI, EI, FOR, NEXT, SRET, IRET, FEND or END
6618FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: STL, MC or MCR can only be used in main program, but it is used elsewhere
6621FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: Numbers of FOR and NEXT instructions do not match
6622FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No NEXT instruction
6623FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No MC instruction
6624FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No MCR instruction
6627FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No STL instruction
6629FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No P or I (interrupt pointer)
6630FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: No SRET or IRET instruction / STL-RET / MC-MCR instructions programmed in the subroutine
6632FX3U, FX3UC, FX3G, FX3GC, FX3SCircuit error: FEND programmed in invalid location
6706FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Device number range or data value for operand of applied instruction exceeds limit
6705FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Operand of applied instruction is inapplicable device
6701FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: No jump destination (pointer) for CJ or CALL instruction
6702FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: CALL instruction nesting level is 6 or more
6708FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: FROM/TO instruction error
6707FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: File register is accessed without parameter setting of file register
6709FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Other (e.g. improper branching)
6730FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Incorrect sampling time (TS)
6762FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Port specified by inverter communication instruction is already used in another communication
6763FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Input (X) specified by a DSZR, DVIT or ZRN instruction is already used in another instruction
6764FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Pulse output number is already used in a positioning instruction or pulse output instruction (PLSY, PWM, etc.)
6771FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Memory cassette is not connected
6772FX3U, FX3UC, FX3G, FX3GC, FX3SOperation error: Memory cassette is protected against writing
6105FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: Watchdog timer error
6101FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: Memory access error
6102FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: Operation circuit error
6103FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: I/O bus error (M8069 = ON)
6104FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: Powered extension unit 24 V failure (M8069 = ON)
6106FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: I/O table creation error (CPU error)
6107FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: System configuration error
6108FX3U, FX3UC, FX3G, FX3GC, FX3SPLC hardware error: Extension bus error
6401FX3U, FX3UC, FX3G, FX3GC, FX3SParameter error: Program sum check error
6402FX3U, FX3UC, FX3G, FX3GC, FX3SParameter error: Memory capacity setting error
6409FX3U, FX3UC, FX3G, FX3GC, FX3SParameter error: Other setting error
1020FX3U, FX3UC, FX3G, FX3GC, FX3SI/O configuration error [M8060(D8060)] (example value)
6301FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 1 [ch1]: Parity, overrun or framing error
6320FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 1 [ch1]: Inverter communication error
6321FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 1 [ch1]: MODBUS communication error
3801FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 2 [ch2]: Parity, overrun or framing error
3806FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 2 [ch2]: Communication time-out detected
3812FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 2 [ch2]: Parallel link character error
3820FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 2 [ch2]: Inverter communication error
3821FX3U, FX3UC, FX3G, FX3GC, FX3SSerial communication error 2 [ch2]: MODBUS communication error
6201FX3U, FX3UC, FX3G, FX3GC, FX3SPLC/PP communication error, Serial communication error 0: Parity, overrun or framing error
2220HFX5U, FX5UC, FX5UJ, FX5SParameter error
2221HFX5U, FX5UC, FX5UJ, FX5SParameter error
2200HFX5U, FX5UC, FX5UJ, FX5SParameter error
1800HFX5U, FX5UC, FX5UJ, FX5SAnnunciator ON
3400HFX5U, FX5UC, FX5UJ, FX5SOperation error
3405HFX5U, FX5UC, FX5UJ, FX5SOperation error
3406HFX5U, FX5UC, FX5UJ, FX5SOperation error
1090HFX5U, FX5UCBattery error
1080HFX5U, FX5UC, FX5UJ, FX5SROM write count error
2120HFX5U, FX5UC, FX5UJ, FX5SMemory card error
2121HFX5U, FX5UC, FX5UJ, FX5SMemory card error
2400HFX5U, FX5UC, FX5UJ, FX5SModule verification error
2440HFX5U, FX5UC, FX5UJ, FX5SModule major error
2450HFX5U, FX5UC, FX5UJ, FX5SModule major error detected
2500HFX5U, FX5UC, FX5UJ, FX5SWDT error
2501HFX5U, FX5UC, FX5UJ, FX5SWDT error
2820HFX5U, FX5UC, FX5UJ, FX5SDevice specification error
2801HFX5U, FX5UC, FX5UJ, FX5SModule specification error
3000HFX5U, FX5UC, FX5UJ, FX5SBoot function execution error
3100HFX5U, FX5UC, FX5UJ, FX5SProgram error
3340HFX5U, FX5UC, FX5UJ, FX5SFOR-NEXT instruction error
3380HFX5U, FX5UC, FX5UJ, FX5SPointer execution error
3600HFX5U, FX5UC, FX5UJ, FX5SOperation error
3C00HFX5U, FX5UC, FX5UJ, FX5SHardware failure
3C20HFX5U, FX5UC, FX5UJ, FX5SMemory error

Each code: causes and first checks

6503: Syntax error: No setting value after OUT T or OUT C / Insufficient number of operands for an applied instruction

Hits OEE: Availability

What it means: A timer or counter coil was programmed without its set value, or an applied instruction is missing one of its operands, so the program fails the syntax check.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • OUT T or OUT C entered without a K constant or D register set value
  • Applied instruction written with too few operands
  • Program edited or converted with an incomplete instruction

First checks, in this order

  1. Read D8065 (syntax error code) and D8069 for the step number (FX3U/FX3UC: D8314 and D8315).
  2. Run Diagnostics, PLC Diagnostics in GX Works2 and jump to the step.
  3. Add the missing set value or operand, write the program in STOP and switch to RUN.

6501: Syntax error: Incorrect combination of instruction, device symbol and device number

Hits OEE: Availability

What it means: An instruction uses a device type or device number it cannot work with, so the PLC rejects the program at the syntax check.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Wrong device letter for the instruction (for example a bit device where a word is needed)
  • Device number that this FX3 model does not have
  • Program written for a different FX series or model

First checks, in this order

  1. Read D8065 and the step number in D8069 (FX3U/FX3UC: D8314 and D8315).
  2. Run a program check and PLC Diagnostics in GX Works2 and go to the reported step.
  3. Confirm the PLC type set in the project matches the real PLC, correct the instruction and write again.

6504: Syntax error: Same label number is used more than once / Same interrupt input or high-speed counter input is used more than once

Hits OEE: Availability

What it means: A pointer label P is defined twice, or two functions (input interrupt, high-speed counter) claim the same physical input.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Duplicate P label after copying program blocks
  • Same X input assigned to an interrupt pointer and a high-speed counter
  • Two high-speed counters that share an input used together

First checks, in this order

  1. Read D8065 and D8069 for the step number.
  2. Search the program for the duplicate P number and renumber one of them.
  3. Check the input assignments of interrupt pointers (I) and high-speed counters (C235 to C255).

6506: Syntax error: Invalid instruction

Hits OEE: Availability

What it means: The program contains an instruction this PLC does not support.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Program made for another FX model or a newer firmware version
  • Wrong PLC type selected in the GX Works2 project
  • Corrupted program in memory or memory cassette

First checks, in this order

  1. Read D8065 and D8069 to find the step.
  2. Check the PLC type and firmware version against the instruction's supported models in the programming manual.
  3. Replace the instruction, or rewrite the program to the PLC.

6505: Syntax error: Device number is out of allowable range

Hits OEE: Availability

What it means: A device number in the program is higher than this PLC model allows.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Device number beyond the model's range (for example a timer, counter or D register that does not exist)
  • Program moved from a larger FX model

First checks, in this order

  1. Read D8065 and D8069 for the step.
  2. Compare the device number with the device range table for your model.
  3. Change the device number and write the program again.

6502: Syntax error: No OUT T or OUT C before setting value

Hits OEE: Availability

What it means: A timer or counter set value appears in the program without the OUT T or OUT C coil in front of it.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Timer or counter coil deleted while its set value stayed
  • Program edited in list (instruction) mode with a missing line

First checks, in this order

  1. Read D8065 and D8069 for the step.
  2. Restore the OUT T or OUT C instruction or remove the orphan set value.

6507: Syntax error: Invalid label number [P]

Hits OEE: Availability

What it means: A pointer label number P is outside the range this PLC accepts.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • P label number above the model's pointer range
  • Program copied from a model with more pointers

First checks, in this order

  1. Read D8065 and D8069 for the step.
  2. Renumber the label inside the allowed P range.

6510: Syntax error: MC nesting number error

Hits OEE: Availability

What it means: Master control (MC) nesting numbers N0 to N7 are used in the wrong order or combination.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • MC nesting levels not in ascending order
  • MC and MCR nesting numbers do not match

First checks, in this order

  1. Read D8065 and D8069 for the step.
  2. Check that each MC N number has the matching MCR N and that nesting goes N0, N1, N2 and so on.

6610: Circuit error: LD, LDI is continuously used 9 times or more

Hits OEE: Availability

What it means: Nine or more LD or LDI instructions follow each other before they are combined, which the PLC cannot process.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Complex parallel branch entered in list mode
  • Program converted from another tool with a broken block structure

First checks, in this order

  1. Read D8066 (circuit error code) and D8069 for the step.
  2. Split the circuit block into smaller blocks so the LD/LDI chain is shorter.
  3. Write the corrected program in STOP and switch to RUN.

6611: Circuit error: More ANB/ORB instructions than LD/LDI instructions

Hits OEE: Availability

What it means: The circuit block has more ANB or ORB instructions than LD or LDI starts, so the block logic does not close correctly.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Extra ANB or ORB entered in list mode
  • A branch start (LD/LDI) deleted during editing

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Count LD/LDI against ANB/ORB in that block and fix the pairing, or redraw the rung in ladder view.

6612: Circuit error: Less ANB/ORB instructions than LD/LDI instructions

Hits OEE: Availability

What it means: The circuit block opens more LD or LDI branches than it closes with ANB or ORB.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Missing ANB or ORB after a parallel or series branch
  • Incomplete edit in list mode

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add the missing ANB/ORB or redraw the rung in ladder view.

6613: Circuit error: MPS is continuously used 12 times or more

Hits OEE: Availability

What it means: Twelve or more MPS (memory push) instructions are stacked, beyond what the PLC stack holds.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Deeply nested output branches in one rung

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Break the rung into several rungs so fewer MPS levels are needed.

6614: Circuit error: No MPS instruction

Hits OEE: Availability

What it means: An MRD or MPP instruction is used without the MPS that should open the branch.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • MPS deleted while MRD or MPP stayed
  • Rung edited in list mode

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Insert the MPS at the branch point or redraw the rung.

6615: Circuit error: No MPP instruction

Hits OEE: Availability

What it means: An MPS branch is opened but never closed with MPP.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Missing MPP at the last output of a branched rung

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add the MPP before the last output of the branch.

6616: Circuit error: No coil between MPS, MRD and MPP, or incorrect combination

Hits OEE: Availability

What it means: A branch built with MPS, MRD and MPP has a section without an output coil, or the three are combined in the wrong order.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Output coil deleted inside a branch
  • MPS, MRD and MPP in the wrong sequence

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Make sure every branch section ends in a coil or applied instruction, and the order is MPS, MRD, MPP.

6617: Circuit error: Instruction below is not connected to bus line: STL, RET, MCR, P, I, DI, EI, FOR, NEXT, SRET, IRET, FEND or END

Hits OEE: Availability

What it means: One of these instructions must start directly at the left power rail, but it is placed behind a contact or inside a branch.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • FEND, END, FOR, NEXT, RET or a label placed after a contact
  • Rung structure broken during editing

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Move the instruction so it connects straight to the bus line.

6618: Circuit error: STL, MC or MCR can only be used in main program, but it is used elsewhere

Hits OEE: Availability

What it means: STL, MC or MCR is placed in an interrupt routine or subroutine, but these instructions are only allowed in the main program.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Step ladder (STL) or master control block moved below FEND into a subroutine or interrupt routine

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Move the STL or MC/MCR block back into the main program, before FEND.

6621: Circuit error: Numbers of FOR and NEXT instructions do not match

Hits OEE: Availability

What it means: The program has a different count of FOR and NEXT instructions, so a loop is left open or closed twice.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Deleted NEXT or extra FOR
  • Loop copied only partly

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Pair every FOR with one NEXT.

6622: Circuit error: No NEXT instruction

Hits OEE: Availability

What it means: A FOR loop is opened but its NEXT instruction is missing.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • NEXT deleted or placed after END/FEND

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add the NEXT that closes the loop.

6623: Circuit error: No MC instruction

Hits OEE: Availability

What it means: An MCR (master control reset) is in the program without the MC that should open it.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • MC deleted while MCR stayed

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add the MC with the same nesting number or delete the orphan MCR.

6624: Circuit error: No MCR instruction

Hits OEE: Availability

What it means: A master control block opened with MC is never closed with MCR.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Missing MCR at the end of the master control block

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add MCR with the matching nesting number.

6627: Circuit error: No STL instruction

Hits OEE: Availability

What it means: A RET (end of step ladder) is used without any STL step before it.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • STL steps deleted while RET stayed
  • RET placed in the wrong program area

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Check the step ladder block starts with STL and ends with RET.

6629: Circuit error: No P or I (interrupt pointer)

Hits OEE: Availability

What it means: A subroutine return or interrupt return is in the program without the P label or I pointer that starts the routine.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • P label or interrupt pointer deleted
  • SRET or IRET placed without its routine

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Add the P label or I pointer at the start of the routine after FEND.

6630: Circuit error: No SRET or IRET instruction / STL-RET / MC-MCR instructions programmed in the subroutine

Hits OEE: Availability

What it means: A subroutine or interrupt routine is missing its SRET or IRET, or it contains a step ladder or master control block that is not allowed there.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • SRET or IRET missing at the end of the routine
  • STL or MC block placed inside a subroutine

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. End each subroutine with SRET and each interrupt routine with IRET; move STL or MC blocks to the main program.

6632: Circuit error: FEND programmed in invalid location

Hits OEE: Availability

What it means: FEND (end of main program) sits in a place where it is not allowed, for example inside a loop or a routine.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • FEND inside a FOR-NEXT loop, a subroutine or a step ladder block

First checks, in this order

  1. Read D8066 and D8069 for the step.
  2. Place FEND once, at the end of the main program, before the subroutines and interrupt routines.

6706: Operation error: Device number range or data value for operand of applied instruction exceeds limit

Warning: act before it becomes a stop

What it means: While running, an applied instruction tried to use a device number or a value outside the allowed range, often because an index register (V or Z) pushed it out of range.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Index register V or Z value too large, for example T500Z with Z = 100 points at T600
  • Data value outside the instruction's limits
  • Table or block move length running past the last device

First checks, in this order

  1. Read D8067 (operation error code) and the step in D8069 (FX3U/FX3UC: D8314 and D8315).
  2. Monitor the index registers and operand values at that step while the machine runs.
  3. Add range checks in the program; M8068 and D8068 keep the first error step latched.

6705: Operation error: Operand of applied instruction is inapplicable device

Warning: act before it becomes a stop

What it means: An applied instruction received a device type it cannot use when it was executed.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Indirect or indexed addressing pointing at the wrong device type
  • Instruction operand set to a device the instruction does not accept

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check the allowed operand devices for that instruction in the programming manual and correct the operand.

6701: Operation error: No jump destination (pointer) for CJ or CALL instruction

Warning: act before it becomes a stop

What it means: A CJ jump or CALL points to a label that is missing, or index addressing pushed the label outside P0 to P4095, or P63 was used with CALL.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Target P label deleted or never written
  • Indexed pointer value out of range
  • P63 used in a CALL (P63 is reserved for jumping to END)

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check the CJ or CALL label and the index register value at that step.
  3. Add the missing label or change the target.

6702: Operation error: CALL instruction nesting level is 6 or more

Warning: act before it becomes a stop

What it means: Subroutines call each other six or more levels deep, beyond the allowed nesting.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Subroutine that calls itself or a chain of nested CALLs

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Flatten the subroutine structure to 5 levels or fewer.

6708: Operation error: FROM/TO instruction error

Warning: act before it becomes a stop

What it means: A FROM or TO instruction could not read or write the buffer memory of a special function unit or block.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Wrong unit number or buffer memory (BFM) number
  • Special block not powered or extension cable loose

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check the unit number and BFM number used in the instruction exist in that module.
  3. Check the extension cables are firmly connected.

6707: Operation error: File register is accessed without parameter setting of file register

Warning: act before it becomes a stop

What it means: The program reads or writes file registers, but file registers are not set up in the PLC parameters.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • File register capacity left at zero in the parameters

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Set the file register capacity in the PLC parameters and write parameters and program again.

6709: Operation error: Other (e.g. improper branching)

Warning: act before it becomes a stop

What it means: An operation error that fits no other code, for example a jump that leaves a block in an invalid way.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Improper branching with CJ or CALL
  • Operand values that are valid in syntax but invalid at run time

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Review the jumps and operand values around that step.

6730: Operation error: Incorrect sampling time (TS)

Warning: act before it becomes a stop

What it means: The PID instruction's sampling time is zero or negative, so the PID instruction stops.[1][2]

PLC response: PLC keeps running, ERROR LED off, PID instruction stopped

Likely causes

  • PID parameter block not initialized
  • Sampling time register overwritten by another part of the program

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check the PID parameter area, especially the sampling time, and make sure the program sets it before the PID instruction runs.

6762: Operation error: Port specified by inverter communication instruction is already used in another communication

Warning: act before it becomes a stop

What it means: An inverter communication instruction targets a serial port that another communication function is already using.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Same channel set for inverter communication and for another protocol (MODBUS, computer link, N:N)
  • Two instructions driving the same port

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check which port each communication instruction uses and the communication settings for that channel.

6763: Operation error: Input (X) specified by a DSZR, DVIT or ZRN instruction is already used in another instruction

Warning: act before it becomes a stop

What it means: A positioning instruction (DSZR, DVIT or ZRN) uses an input that is already taken by another function, or the DVIT interrupt input is set outside its range.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Input also used as an input interrupt, a high-speed counter (C235 to C255), a pulse catch (M8170 to M8177) or an SPD instruction
  • Wrong interrupt input setting for DVIT in D8336

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Check the input is used by only one function.
  3. Check D8336 for the DVIT interrupt input setting.

6764: Operation error: Pulse output number is already used in a positioning instruction or pulse output instruction (PLSY, PWM, etc.)

Warning: act before it becomes a stop

What it means: Two positioning or pulse output instructions try to drive the same pulse output at the same time.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • PLSY, PWM, DRVI, DRVA or a similar instruction active on an output that another one is already driving
  • Missing interlock between two motion sequences

First checks, in this order

  1. Read D8067 and D8069 for the step.
  2. Interlock the instructions so only one drives each pulse output at a time, and wait for the busy flag to clear before starting the next.

6771: Operation error: Memory cassette is not connected

Warning: act before it becomes a stop

What it means: The program tried to access the memory cassette, but the PLC does not detect one.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Memory cassette missing or not fully seated

First checks, in this order

  1. Power off, then check the memory cassette is fitted straight and fully pressed in.
  2. Read D8067 and D8069 for the step that accessed it.

6772: Operation error: Memory cassette is protected against writing

Warning: act before it becomes a stop

What it means: Data was sent to the flash memory cassette while its write-protect switch was on.[1][2]

PLC response: PLC keeps running, ERROR LED off

Likely causes

  • Protect switch on the memory cassette set to ON

First checks, in this order

  1. Power off, set the cassette protect switch to OFF and repeat the write.

6105: PLC hardware error: Watchdog timer error

Hits OEE: Availability

What it means: One program scan took longer than the watchdog time in D8000, so the PLC stopped itself.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Long loops (FOR-NEXT), many repeated instructions or large block moves in one scan
  • Input interrupts or pulse catch inputs chattering many times per scan
  • High-speed counter input frequency above specification
  • Electrical noise on the PLC power or ground

First checks, in this order

  1. Switch to STOP and cycle power. If the ERROR LED goes off, compare the maximum scan time D8012 with the watchdog setting D8000.
  2. Shorten the scan, add WDT instructions inside long sections, or raise D8000 with a MOV in the program.
  3. If the LED stays lit, power the PLC from another supply on a bench to test for noise, then check grounding and add a line noise filter.

6101: PLC hardware error: Memory access error

Hits OEE: Availability

What it means: The PLC could not access its program memory or the memory cassette correctly.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Memory cassette loose or not fitted correctly
  • Internal memory fault in the main unit

First checks, in this order

  1. Read D8061.
  2. Power off and reseat the memory cassette, if one is used.
  3. If it continues, or no cassette is fitted, the PLC may be faulty: contact Mitsubishi Electric.

6102: PLC hardware error: Operation circuit error

Hits OEE: Availability

What it means: The PLC's internal operation circuit reported a fault, which is often caused by electrical noise.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Noise on the power supply or ground
  • Internal hardware fault

First checks, in this order

  1. Read D8061.
  2. Power the PLC from a different supply on a bench. If the ERROR LED goes off, check grounding, wiring routes and install location, and fit a noise filter on the power line.
  3. If the LED stays lit, contact Mitsubishi Electric.

6103: PLC hardware error: I/O bus error (M8069 = ON)

Hits OEE: Availability

What it means: With I/O bus checking on (M8069 set), the PLC found a fault on the extension bus to I/O or special units.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Extension cable loose or damaged
  • Faulty extension unit or block

First checks, in this order

  1. Read D8061.
  2. Power off and reseat every extension cable.
  3. Remove extension units one at a time to find the faulty one.

6104: PLC hardware error: Powered extension unit 24 V failure (M8069 = ON)

Hits OEE: Availability

What it means: With M8069 set, the 24 V DC from a powered extension unit or extension power supply has failed; M8009 turns on and D8009 shows the first affected I/O number.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Powered extension unit switched off or its supply failed
  • 24 V DC service supply overloaded or shorted

First checks, in this order

  1. Read D8061 and D8009.
  2. Check the supply to the powered extension unit and the load on its 24 V output.
  3. Check extension cables.

6106: PLC hardware error: I/O table creation error (CPU error)

Hits OEE: Availability

What it means: At power-on the PLC could not build its I/O table, for example because a powered extension unit had no 24 V for 10 seconds or more after the main unit powered up.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Powered extension unit powered on too late or without 24 V
  • CC-Link/LT I/O assignment disabled at power-on (FX3UC-32MT-LT(-2))

First checks, in this order

  1. Read D8061.
  2. Power the extension units at the same time as, or before, the main unit.
  3. Check extension cables and supplies.

6107: PLC hardware error: System configuration error

Hits OEE: Availability

What it means: Too many special function units or blocks are connected, or a unit's own connection limit is exceeded.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit

Likely causes

  • Number of special function units/blocks above the system limit
  • A unit type connected more times than it allows

First checks, in this order

  1. Read D8061.
  2. Count the special units and blocks and compare with the system configuration limits in the hardware manual.

6108: PLC hardware error: Extension bus error

Hits OEE: Availability

What it means: Communication on the extension bus failed.[1][2]

PLC response: PLC is forced to STOP, ERROR LED lit (when M8484 is on)

Likely causes

  • Extension cable loose, damaged or routed near power wiring
  • Faulty extension unit or block

First checks, in this order

  1. Read D8061.
  2. Power off and reseat the extension cables.
  3. Note: the PLC only stops on this error when M8484 is on.

6401: Parameter error: Program sum check error

Hits OEE: Availability

What it means: The checksum of the program or parameters in memory does not match, so the PLC will not run it.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Program or parameter write interrupted
  • Memory cassette using a function the PLC firmware does not support (permanent PLC lock, block password read protection, FX3U-FLROM-1M on older versions)

First checks, in this order

  1. Read D8064.
  2. Switch to STOP, set the parameters correctly and write program and parameters again.
  3. With a memory cassette, check the PLC version supports the functions used.

6402: Parameter error: Memory capacity setting error

Hits OEE: Availability

What it means: The program memory size set in the parameters is not valid for this PLC or memory.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Memory capacity set larger than the installed memory supports
  • Project made for another model

First checks, in this order

  1. Read D8064.
  2. Correct the memory capacity in the PLC parameters and write again.

6409: Parameter error: Other setting error

Hits OEE: Availability

What it means: A parameter setting is invalid but fits none of the specific parameter error codes.[1][2]

PLC response: PLC will not run (stays in STOP), ERROR LED flashing

Likely causes

  • Invalid value in the PLC parameters

First checks, in this order

  1. Read D8064.
  2. Switch to STOP, review all PLC parameters and write them again.

1020: I/O configuration error [M8060(D8060)] (example value)

Warning: act before it becomes a stop

What it means: The program uses I/O numbers that are not physically present. D8060 holds the first missing I/O: the last three digits are the device number and the first digit is 1 for an input (X) or 0 for an output (Y), so 1020 means X020 and later are not connected.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Program uses X or Y numbers beyond the installed I/O
  • Extension unit or block missing, unpowered or not detected

First checks, in this order

  1. Read D8060 and decode the I/O number (device number range 10 to 337 on FX3U/FX3UC, 10 to 177 on FX3G/FX3GC).
  2. Check the extension unit wiring and cables, add the missing unit, or change the program to existing I/O.

6301: Serial communication error 1 [ch1]: Parity, overrun or framing error

Warning: act before it becomes a stop

What it means: The serial port on channel 1 received data with a parity, overrun or framing error.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Baud rate, data bits, parity or stop bits differ between the PLC and the other device
  • Wrong or loose RS-485/RS-232C wiring
  • Noise on the communication cable

First checks, in this order

  1. Read D8063.
  2. Check the channel 1 communication settings match the other device.
  3. Check the cable wiring, shielding and termination.

6320: Serial communication error 1 [ch1]: Inverter communication error

Warning: act before it becomes a stop

What it means: Inverter communication on channel 1 failed.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Inverter station number or communication settings wrong
  • RS-485 wiring fault to the inverter
  • Inverter not powered

First checks, in this order

  1. Read D8063.
  2. Check the inverter's communication parameters and station number against the PLC settings.
  3. Check the RS-485 wiring.

6321: Serial communication error 1 [ch1]: MODBUS communication error

Warning: act before it becomes a stop

What it means: MODBUS serial communication on channel 1 failed.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • MODBUS settings (slave address, baud rate, format) mismatch
  • Slave not responding
  • Wiring fault

First checks, in this order

  1. Read D8063.
  2. Check the MODBUS settings on both sides and the program that runs the MODBUS instructions.
  3. Check the cable.

3801: Serial communication error 2 [ch2]: Parity, overrun or framing error

Warning: act before it becomes a stop

What it means: The serial port on channel 2 received data with a parity, overrun or framing error.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Communication settings differ between the PLC and the other device
  • Wrong or loose wiring
  • Noise on the communication cable

First checks, in this order

  1. Read D8438.
  2. Check the channel 2 settings match the connected device.
  3. Check the cable wiring and shielding.

3806: Serial communication error 2 [ch2]: Communication time-out detected

Warning: act before it becomes a stop

What it means: Channel 2 waited for a reply from the other station and the time limit ran out.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Other device powered off or not responding
  • Broken or disconnected cable
  • Time-out setting too short

First checks, in this order

  1. Read D8438.
  2. Check the other device is on and responding.
  3. Check the cable and the time-out setting.

3812: Serial communication error 2 [ch2]: Parallel link character error

Warning: act before it becomes a stop

What it means: The parallel link on channel 2 received a bad character.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Parallel link settings differ between the two PLCs
  • Wiring or noise problem on the link cable

First checks, in this order

  1. Read D8438.
  2. Check the parallel link program and settings on both PLCs.
  3. Check the link cable.

3820: Serial communication error 2 [ch2]: Inverter communication error

Warning: act before it becomes a stop

What it means: Inverter communication on channel 2 failed.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Inverter communication settings or station number wrong
  • RS-485 wiring fault
  • Inverter not powered

First checks, in this order

  1. Read D8438.
  2. Check the inverter communication parameters and the PLC channel 2 settings.
  3. Check the RS-485 wiring.

3821: Serial communication error 2 [ch2]: MODBUS communication error

Warning: act before it becomes a stop

What it means: MODBUS serial communication on channel 2 failed.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • MODBUS settings mismatch
  • Slave not responding
  • Wiring fault

First checks, in this order

  1. Read D8438.
  2. Check the MODBUS settings and the program on channel 2.
  3. Check the cable.

6201: PLC/PP communication error, Serial communication error 0: Parity, overrun or framing error

Warning: act before it becomes a stop

What it means: The programming port link to a programming tool or panel received corrupted data.[1][2]

PLC response: PLC keeps its RUN or STOP state, ERROR LED off

Likely causes

  • Programming cable unplugged and replugged while monitoring
  • Faulty or loose programming cable

First checks, in this order

  1. Read D8062.
  2. Check the cable between the PC or programming panel and the PLC, then reconnect.

2220H: Parameter error

Hits OEE: Availability

What it means: The parameter data in the CPU module is corrupted, so the CPU will not start the program.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Corrupted parameter data in the CPU module

First checks, in this order

  1. Read SD0, or open module diagnostics in GX Works3 to see the parameter information.
  2. Write the full project (parameters and program) to the CPU again, then reset or cycle power.

2221H: Parameter error

Hits OEE: Availability

What it means: A parameter value is out of range, or a parameter turns on a function this CPU firmware does not support.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Parameter value outside its range
  • Function or I/O count not supported by the CPU firmware version

First checks, in this order

  1. Check the parameter number shown in GX Works3 module diagnostics and correct the value.
  2. Check the CPU firmware version and update it if the function needs a newer one.
  3. Write the project again.

2200H: Parameter error

Hits OEE: Availability

What it means: The CPU module cannot find its parameter file.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Parameters never written, or deleted from the CPU

First checks, in this order

  1. Write the full project including parameters to the CPU, then reset or cycle power.

1800H: Annunciator ON

Warning: act before it becomes a stop

What it means: The program turned on an annunciator (F device) with SET F or OUT F, usually as a user-defined machine alarm.[3]

PLC response: CPU module keeps running (continuation error)

Likely causes

  • A machine condition the programmer flagged as an alarm

First checks, in this order

  1. Read the annunciator number in GX Works3 module diagnostics and find that F number in the program.
  2. Fix the machine condition, then reset the annunciator with RST F.

3400H: Operation error

Warning: act before it becomes a stop

What it means: An applied instruction tried to divide by zero.[3]

PLC response: Continuation error by default; can be set to stop in CPU Parameter, RAS Setting

Likely causes

  • Divisor register at 0 when the division runs

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step; SD8067 also holds the operation error code.
  2. Add a check so the division only runs when the divisor is not 0.

3405H: Operation error

Warning: act before it becomes a stop

What it means: An applied instruction received input data outside its allowed range.[3]

PLC response: Continuation error by default; can be set to stop in CPU Parameter, RAS Setting

Likely causes

  • Operand value out of range at run time
  • Index or pointer value pushing data out of range

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Review the data given to that instruction and add range limits.

3406H: Operation error

Warning: act before it becomes a stop

What it means: The result of an applied instruction would land outside the allowed device range.[3]

PLC response: Continuation error by default; can be set to stop in CPU Parameter, RAS Setting

Likely causes

  • Destination device plus data length runs past the last device

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Change the destination device or the transfer length.

1090H: Battery error

Warning: act before it becomes a stop

What it means: The battery voltage is low, or a battery-backed device reported an error.[3][4]

PLC response: CPU module keeps running (continuation error), BAT LED flashing

Likely causes

  • Battery near end of life (FX3U-32BL, standard life about 5 years at 25 C)
  • Battery connector loose

First checks, in this order

  1. Check the BAT LED (flashing) and the battery connection.
  2. Read the battery voltage in SD8005; SM51 and SM52 turn on at low voltage.
  3. Replace the battery soon; memory is held for about one month after the LED turns on.

1080H: ROM write count error

Warning: act before it becomes a stop

What it means: The built-in data memory has been written more than 20,000 times.[3]

PLC response: CPU module keeps running (continuation error)

Likely causes

  • Very frequent writes to the CPU's data memory over the module's life

First checks, in this order

  1. Replace the CPU module.
  2. Review the application so it does not write to the data memory so often.

2120H: Memory card error

Warning: act before it becomes a stop

What it means: The CPU detected an SD memory card error, possibly because the card was pulled out without being disabled first.[3]

PLC response: Continuation error by default; stops if detected at startup or if set to stop in parameters

Likely causes

  • SD card removed without disabling it
  • Card or slot fault

First checks, in this order

  1. Check the SD card connection and reinsert it.
  2. Replace the card if the error stays; the CPU or card may be faulty.

2121H: Memory card error

Warning: act before it becomes a stop

What it means: The CPU detected an SD memory card error, possibly because the card is not formatted correctly.[3]

PLC response: Continuation error by default; stops if detected at startup or if set to stop in parameters

Likely causes

  • SD card not formatted for the CPU

First checks, in this order

  1. Format the SD card (with the CPU or GX Works3).
  2. Replace the card if the error stays.

2400H: Module verification error

Hits OEE: Availability

What it means: A connected extension module is off or lost its connection, or the internal bus timed out.[3]

PLC response: Stop error by default; can be set to continue in CPU Parameter, RAS Setting

Likely causes

  • Extension module power off
  • Extension cable loose or damaged
  • Noise on the bus

First checks, in this order

  1. Check every connected module is powered.
  2. Reseat the extension cables and check noise measures.
  3. Replace the module or cable if it repeats. SM61 stays on until the CPU is reset or cycled.

2440H: Module major error

Hits OEE: Availability

What it means: The CPU could not complete the start-up communication with a connected module.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Extension cable fault
  • CPU firmware too old for the connected module
  • Faulty connected module

First checks, in this order

  1. Check the extension cables.
  2. Check the CPU firmware version supports that module.
  3. Clear the module's own error from the GX Works3 system monitor, then replace the module if it repeats.

2450H: Module major error detected

Hits OEE: Availability

What it means: An intelligent function module reported a major error to the CPU.[3]

PLC response: Stop error by default; can be set to continue in CPU Parameter, RAS Setting

Likely causes

  • Fault in the intelligent function module
  • Noise or extension cable problem

First checks, in this order

  1. Find the module number in GX Works3 module diagnostics (system configuration information) and check that module's manual.
  2. Check noise measures and extension cables.
  3. Reset the CPU and run again; a repeat points to a module hardware fault.

2500H: WDT error

Hits OEE: Availability

What it means: The first scan took longer than the execution monitor (watchdog) time, or a fixed-cycle interrupt program ran longer than its interval.[3][4]

PLC response: CPU module stops (stop error)

Likely causes

  • Heavy initial processing in the first scan
  • Fixed-cycle interrupt program too long for its interval

First checks, in this order

  1. Check the execution monitor time setting and the program.
  2. Shorten the interrupt program or lengthen its interval.
  3. Compare the maximum scan time in SD524 and SD525 with the monitor setting.

2501H: WDT error

Hits OEE: Availability

What it means: A scan after the first one took longer than the execution monitor (watchdog) time, or a fixed-cycle interrupt program ran longer than its interval.[3][4]

PLC response: CPU module stops (stop error)

Likely causes

  • Long loops or heavy instructions in one scan
  • Input interrupts or pulse catch inputs switching many times per scan
  • High-speed counter input above its specified frequency

First checks, in this order

  1. Compare the maximum scan time (SD524, SD525) with the scan time monitoring setting.
  2. Shorten the program path or add WDT instructions inside long sections.
  3. Check interrupt and high-speed counter inputs for chatter or excess frequency.

2820H: Device specification error

Warning: act before it becomes a stop

What it means: A device used as an instruction operand is outside the allowed device range.[3]

PLC response: CPU module keeps running (continuation error)

Likely causes

  • Indexed or indirect device address out of range

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Check the device range and correct the program.

2801H: Module specification error

Warning: act before it becomes a stop

What it means: An instruction names a module number that is not fitted, or uses wrong devices as operands.[3]

PLC response: Continuation error by default; can be set to stop in CPU Parameter, RAS Setting

Likely causes

  • Wrong module number in FROM, TO or a module instruction
  • Module removed but program unchanged

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Set the correct module number and check the operand device ranges.

3000H: Boot function execution error

Hits OEE: Availability

What it means: The CPU found an error in the boot file on the SD memory card.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Damaged or wrong boot file on the SD card

First checks, in this order

  1. Replace the boot file on the SD card with a correct one.
  2. Cycle the PLC power.

3100H: Program error

Hits OEE: Availability

What it means: The program contains an instruction the CPU cannot use or decode, or instructions that cannot be used together.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Instruction not supported by this CPU or firmware
  • Unicode strings used where they are not supported
  • Program corrupted by noise

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Check noise measures.
  3. Write the program again and reset the CPU; a repeat points to CPU hardware.

3340H: FOR-NEXT instruction error

Hits OEE: Availability

What it means: FOR and NEXT instructions are not paired correctly.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • FOR and NEXT executed a different number of times
  • Jump out of a FOR loop

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Make sure every FOR has a NEXT and jumps do not leave the loop.

3380H: Pointer execution error

Hits OEE: Availability

What it means: A jump or call points to a label that is missing.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • Jump target pointer deleted or misnumbered

First checks, in this order

  1. Use Error Jump in GX Works3 module diagnostics to find the step.
  2. Set the correct jump destination.

3600H: Operation error

Warning: act before it becomes a stop

What it means: A communication or high-speed I/O instruction uses a channel that has no matching parameters, or the board, adapter or module for that channel is not fitted.[3]

PLC response: Continuation error by default; can be set to stop in CPU Parameter, RAS Setting

Likely causes

  • Channel not set up in the parameters
  • Communication board, adapter or module missing

First checks, in this order

  1. Check the parameter settings for the channel used by the instruction.
  2. Check the right board, adapter or module is installed on that channel.

3C00H: Hardware failure

Hits OEE: Availability

What it means: The CPU module detected a hardware failure.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • CPU module hardware fault
  • Noise (if it clears after a reset)

First checks, in this order

  1. Reset the CPU module and switch to RUN.
  2. If the same error returns, contact Mitsubishi Electric about the CPU module.

3C20H: Memory error

Hits OEE: Availability

What it means: The CPU module detected a memory error.[3]

PLC response: CPU module stops (stop error)

Likely causes

  • CPU memory fault
  • Corrupted memory contents

First checks, in this order

  1. Reset the CPU module and switch to RUN.
  2. If it returns, initialize the memory and write the project again.
  3. If the memory still cannot be recovered, contact Mitsubishi Electric.

Log it, or the same alarm comes back

A PLC error that stops the CPU stops every output it drives, so in OEE terms it is a breakdown, and often of a whole machine. Log the error code, the machine and the minutes lost, and whether it appeared after a program change.

  • Reason code: the code plus the machine, for example FX3U 6503, carton erector PLC, so an error that returns after every program download shows up as a pattern.
  • Loss bucket: a stop that needs a reset, a part or a technician is an availability loss (breakdown). Stops shorter than your micro-stop threshold go to performance (minor stops). The six big losses page shows where each one belongs.
  • What to count: how often each code fires per week and the minutes it costs. Two numbers a week turn "that alarm comes up sometimes" into a ranked list. The MTBF and MTTR calculator and the downtime cost calculator do the math.

The free OEE tracker logs each stop with its reason and turns the week into a downtime Pareto, so the code that costs the most rises to the top.

To pull machine states and counts from a PLC like this into OEE, fabrico.io has a guide to PLC connectivity for OEE monitoring. An error code tells you what the PLC detected, not what happened at the machine in the seconds before. Fabrico's camera-based OEE software syncs camera video with machine data, so you can replay the moment the machine stopped next to the error.

Frequently asked questions

What does error 6503 mean on a Mitsubishi FX3U?

6503 is a syntax error stored in D8065. Mitsubishi lists two causes: an OUT T or OUT C (timer or counter coil) without its set value, or an applied instruction with too few operands. The PLC will not run and the ERROR LED flashes. Read the step number in D8069 (on FX3U/FX3UC also D8314 and D8315) or run Diagnostics, PLC Diagnostics in GX Works2, add the missing set value or operand, write the program and switch STOP to RUN.

Why is the ERROR LED flashing on my FX3U?

A flashing ERROR LED means a parameter error (64xx in D8064), a syntax error (65xx in D8065) or a circuit error (66xx in D8066), and the PLC stays in STOP. A steady lit ERROR LED is different: it means a watchdog timer error (6105) or a PLC hardware error (61xx in D8061). Check D8004 first: it tells you which error register to read, for example 8065 points to D8065.

How do I clear an error on a Mitsubishi FX3U?

Fix the cause first. Then switch the PLC from STOP to RUN: this clears M8060, M8061 and M8064 to M8067 and their error registers. Cycling power also resets M8004, but it turns back on if the cause is still there. The operation error latch M8068 (and D8068) is not cleared by STOP to RUN; reset it with the device monitor in GX Works2. For a steady lit ERROR LED (61xx hardware error), Mitsubishi's first step is to switch to STOP and cycle power, then look for noise or a hardware fault if it stays lit.

Where is the error code stored on an FX3U or FX3G?

M8004 turns on when any error flag M8060, M8061 or M8064 to M8067 is on, and D8004 holds the lowest flag number. The codes sit in D8060 (I/O configuration, for example 1020 means X020 and up are missing), D8061 (hardware, 61xx), D8062 (programming port, 62xx), D8063 (serial channel 1, 63xx), D8438 (serial channel 2, 38xx), D8064 (parameter, 64xx), D8065 (syntax, 65xx), D8066 (circuit, 66xx) and D8067 (operation, 67xx). 0000 means no error.

How do I check and clear an error on a Mitsubishi FX5U?

On the FX5 (iQ-F) the CPU turns on SM0 and SM1 and stores the latest code in SD0, with up to 16 current codes in SD10 to SD25. Codes are 4-digit hex, for example 2220H (parameter error, stops the CPU) or 3400H (division by zero, a continuation error by default). Open module diagnostics in GX Works3 to see the cause and jump to the program step. Continuation errors clear once you fix the cause and switch SM50 off and on, or from module diagnostics. Stop errors clear after you fix the cause and reset the CPU or cycle power.

Sources

Codes and meanings follow the manufacturer documentation below. Wording is ours, simplified for the shop floor; the manual for your exact model and software version wins if it differs.

  1. Mitsubishi Electric FX3S/FX3G/FX3GC/FX3U/FX3UC Series Programming Manual, Basic & Applied Instruction Edition, JY997D16601 Revision R, April 2021
  2. Mitsubishi Electric FX3U Series User's Manual, Hardware Edition, JY997D16501 Revision Y, June 2024
  3. Mitsubishi Electric MELSEC iQ-F FX5 User's Manual (Application), JY997D55401 Revision AG, April 2026
  4. Mitsubishi Electric MELSEC iQ-F FX5S/FX5UJ/FX5U/FX5UC User's Manual (Hardware), SH(NA)-082452ENG-Q, August 2026
  5. Mitsubishi Electric FA FAQ No. 22280, How to reset M8004 (Error occurrence), released 2018-08-30

See the moment the machine stopped

Fabrico, a light MES with computer vision, syncs camera footage with machine and timeline data, so you can replay the exact moment the machine stopped and review the cause next to the PLC's error. Cameras also catch short stops and a line that is switched on but not producing, the losses nobody writes down. Machines connect through a PLC, a Fabrico IoT device or an AI camera.

Book a Fabrico demo
The lookup stays free.

Related: MicroLogix fault codes · Mitsubishi MELSERVO alarms · Mitsubishi FR alarm codes · PLC and controls troubleshooting

Other alarm code lookups: FANUC: servo amplifier · spindle amplifier · robot SRVO | Siemens: MICROMASTER 440 · MASTERDRIVES · SINAMICS G120 · SINAMICS V90 servo · SINAMICS V20 | ABB: ACS550 drive · ACS800 drive · robot IRC5 | Okuma: OSP alarms | Haas: alarm lookup | Mitsubishi: MELSERVO servo · FR-D700 and FR-E700 inverters | Allen-Bradley: Kinetix and Ultra3000 · MicroLogix · PowerFlex 753 and 755 · PowerFlex 70 and 700 · 1336 and 1305 | Yaskawa: V1000 drive · SERVOPACK servo · Motoman robot | Delta: VFD drives · ASDA servo | Eaton: DC1 and SVX9000 drives | Control Techniques: Commander SK and M200 | Kollmorgen: AKD servo | Lenze: SMVector, 8200, 8400, i500 | Magnetek: IMPULSE and DSD 412 | Omron: MX2 and RX inverters · G5 and 1S servo | WEG: CFW500, CFW300, CFW11, CFW08 | Hitachi: WJ200 inverter | Toshiba: E3, Q9, P9, S15, nC3, AS3 | AutomationDirect: GS10, GS20, GS4, GS1 to GS3 | SANYO DENKI: SANMOTION servo | Parker: SSD 590+, 690+, AC10 | KEB: COMBIVERT F5 | Epson: robot RC700, RC90 | B&R: ACOPOS servo | Schneider Electric: Lexium 32 servo | Bosch Rexroth: IndraDrive | Vacon: NX, VACON 100 and 20 | Fuji Electric: FRENIC-Mini, Multi, Ace, MEGA | LS Electric: S100, G100, iG5A | Panasonic: MINAS A6 and A5 servo | Invertek: Optidrive E3, P2, Eco | SEW-Eurodrive: MOVITRAC B and MOVIDRIVE B

Related tools: full troubleshooting directory · OEE calculator · downtime cost · MTBF / MTTR · glossary

OEE essentials: OEE calculator · What is OEE · How to improve OEE · Six big losses · Availability · TEEP · Cost of downtime · OEE benchmark · Free OEE tracker · Free OEE software · How to track downtime · How to implement OEE · Light MES vs full MES