Mitsubishi MELSERVO Alarm Codes (MR-J4, J3, J2S)

How alarms show on the amplifier

MR-J2S-A and MR-J3-A show an alarm on the 5-digit front display as AL. plus the number (for example AL.33 overvoltage).

The current alarm flickers, any other screen can still be called up while the decimal point of the fourth digit keeps flickering, and alarm mode also shows the alarm history (the last six alarms) and the number of a faulty parameter (MR-J2S-A section 6.4, MR-J3-A section 6.5).

An alarm turns off the ALM (trouble) output and stops the motor, while a warning (AL.92 to AL.EA, plus AL.EC and AL.ED on J3) lets operation continue, except AL.E6 and AL.EA which put the amplifier into servo-off, and a warning clears itself once its cause is removed (J2S section 10.2.1, J3 sections 9.1 and 9.3).

After removing the cause, an alarm is cleared by turning on RES (Reset), pressing SET on the current alarm screen, or switching power off and on, but each code has its own allowed methods: encoder, memory and board alarms clear only with a power cycle, and AL.30, AL.45, AL.46, AL.50 and AL.51 need about 30 minutes of cooling first.

MR-J4-A adds 'Occurred Alarm Reset' in the MR Configurator2 Alarm Display window, and MR-J4-B is reset by the controller's reset command, a CPU reset or a power cycle (MR-J4 Troubleshooting manual section 1.1).

MR-J4 adds a detail number after the dot (AL.10.1 control circuit voltage drop, AL.10.2 main circuit voltage drop), does not record AL.37 or warnings (except AL.F0) in the alarm history, and the history is cleared with parameter No.16 on J2S, PC18 on J3 and J4-A, or PC21 on J4-B, set to 1 and then power cycled (Mitsubishi FA FAQ 23650 for J4).

Safety first.

All three manuals give the same warning: before wiring or inspection, switch the power off, wait 15 minutes or more until the charge lamp goes out, then confirm with a voltage tester that the voltage between P and N (P+ and N- on MR-J4) is safe, and always check the charge lamp from the front of the amplifier.

MR-J3 drive units of 30 kW or more and MR-J4 converter units need 20 minutes. The MR-J4 manual adds that the heat sink, regenerative resistor and servo motor can stay hot for some time after power off, and that the run signal must be off before an alarm is reset so the machine cannot restart suddenly.

Follow lockout/tagout and your site's safe-work procedures, and check the manual for your exact model before you act.

Quick lookup

Alarms switch off the ALM output and stop the motor, so in OEE they are availability losses; warnings let the servo keep running until you act.

Type a code or a word (encoder, overload, overcurrent, regenerative) to filter the list. Click a code for its causes and first checks.

Find a code or a word

43 codes

Code	Type	Name
AL.10	Alarm	Undervoltage
AL.12	Alarm	Memory error 1 (RAM)
AL.13	Alarm	Clock error
AL.15	Alarm	Memory error 2 (EEP-ROM)
AL.16	Alarm	Encoder initial communication error 1 (MR-J4) / Encoder error 1 (MR-J2S, MR-J3)
AL.17	Alarm	Board error
AL.19	Alarm	Memory error 3 (Flash-ROM)
AL.1A	Alarm	Servo motor combination error (MR-J4) / Motor combination error (MR-J2S, MR-J3)
AL.20	Alarm	Encoder normal communication error 1 (MR-J4) / Encoder error 2 (MR-J2S, MR-J3)
AL.21	Alarm	Encoder normal communication error 2 (MR-J4) / Encoder error 3 (MR-J3)
AL.24	Alarm	Main circuit error
AL.25	Alarm	Absolute position erased (MR-J4) / Absolute position erase (MR-J2S, MR-J3)
AL.30	Alarm	Regenerative error
AL.31	Alarm	Overspeed
AL.32	Alarm	Overcurrent
AL.33	Alarm	Overvoltage
AL.34	Alarm	SSCNET receive error 1
AL.35	Alarm	Command frequency error (MR-J4) / Command pulse frequency error (MR-J2S, MR-J3)
AL.37	Alarm	Parameter error
AL.45	Alarm	Main circuit device overheat
AL.46	Alarm	Servo motor overheat
AL.47	Alarm	Cooling fan error (MR-J4) / Cooling fan alarm (MR-J3)
AL.50	Alarm	Overload 1
AL.51	Alarm	Overload 2
AL.52	Alarm	Error excessive
AL.8A	Alarm	Serial communication time-out error (MR-J2S, MR-J3) / USB communication time-out error, serial communication time-out error, Modbus RTU communication time-out error (MR-J4)
AL.8E	Alarm	Serial communication error (MR-J2S, MR-J3) / USB communication error, serial communication error, Modbus RTU communication error (MR-J4)
88888	Alarm	Watchdog
AL.92	Warning	Battery cable disconnection warning (MR-J3, MR-J4) / Open battery cable warning (MR-J2S)
AL.96	Warning	Home position setting warning
AL.99	Warning	Stroke limit warning
AL.9F	Warning	Battery warning
AL.E0	Warning	Excessive regeneration warning (MR-J3, MR-J4) / Excessive regenerative warning (MR-J2S)
AL.E1	Warning	Overload warning 1 (MR-J3, MR-J4) / Overload warning (MR-J2S)
AL.E3	Warning	Absolute position counter warning
AL.E5	Warning	ABS time-out warning
AL.E6	Warning	Servo forced stop warning (MR-J4) / Servo emergency stop warning (MR-J2S, MR-J3)
AL.E7	Warning	Controller forced stop warning
AL.E8	Warning	Cooling fan speed reduction warning
AL.E9	Warning	Main circuit off warning
AL.EA	Warning	ABS servo-on warning
AL.EC	Warning	Overload warning 2
AL.ED	Warning	Output watt excess warning

Each code: causes and first checks

AL.10: Undervoltage

Hits OEE: Availability

What it means: The control circuit or main circuit supply voltage dropped below the level the amplifier needs.^[1]^[2]^[3]

Also on: Same number and name on MR-J2S, MR-J3 and MR-J4; MR-J4 splits it into AL.10.1 (control circuit power) and AL.10.2 (main circuit power).

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Supply voltage too low, or a supply too small that sags when the motor accelerates
Instantaneous power failure of 60 ms or longer (on MR-J4 the time is set by Pr. PA20 and Pr. PF25)
Control circuit or main circuit power wiring loose or wrong, or on MR-J4 the P3 to P4 connection removed
Faulty parts inside the amplifier

First checks, in this order

Measure the supply at the amplifier: the alarm level is 160 V AC on 200 V class models, 280 V AC on 400 V class and 83 V AC on 100 V class
Check the control circuit and main circuit power wiring, and on MR-J4 the wiring between P3 and P4
If it trips while accelerating, check the bus voltage (200 V DC limit on 200 V class, 380 V DC on 400 V class) and lengthen the acceleration time or add supply capacity
When switching off and on again, wait until the amplifier display has gone dark; on J2S and J3, if AL.10 appears with only the control power cables connected, replace the amplifier

AL.12: Memory error 1 (RAM)

Hits OEE: Availability

What it means: A RAM memory part inside the amplifier has failed.^[1]^[2]^[3]

Also on: Same on MR-J2S (Memory error 1), MR-J3 and MR-J4 (detail numbers AL.12.1 to AL.12.6).

How it clears: Power off and on only

Likely causes

Faulty part in the amplifier
Electrical noise near the amplifier (MR-J4)

First checks, in this order

Disconnect every cable except the control circuit power and switch on: if AL.12 comes back, replace the amplifier
If it does not come back, check the power supply for noise and the connectors for shorts, then reconnect one cable at a time

AL.13: Clock error

Hits OEE: Availability

What it means: A clock fault on the amplifier's printed board, or on MR-J4-B a clock error sent from the controller.^[1]^[2]^[3]

Also on: Same on MR-J2S, MR-J3 and MR-J4 (AL.13.1 to AL.13.3); the controller clock cause applies to MR-J4-B type amplifiers.

How it clears: Power off and on only

Likely causes

Faulty printed board part in the amplifier
MR-J4-B: clock error coming from the servo system controller, or the next axis amplifier is faulty
Electrical noise near the amplifier

First checks, in this order

Disconnect every cable except the control circuit power and switch on: if AL.13 comes back, replace the amplifier
On MR-J4-B, check whether the alarm follows the controller or the next axis amplifier
Check the power supply for noise and the connectors for shorts

AL.15: Memory error 2 (EEP-ROM)

Hits OEE: Availability

What it means: The EEP-ROM that stores parameters in the amplifier has failed.^[1]^[2]^[3]

Also on: Same on MR-J2S (Memory error 2), MR-J3 and MR-J4 (AL.15.1 at power on, AL.15.2 during operation).

How it clears: Power off and on only

Likely causes

Faulty EEP-ROM in the amplifier
EEP-ROM written more than 100,000 times, for example parameters or point tables rewritten by a PLC or PC on every cycle
Electrical noise near the amplifier (MR-J4)

First checks, in this order

Disconnect every cable except the control circuit power and switch on: if AL.15 comes back, replace the amplifier
Check whether the controller or a PC writes parameters very often, and change the program to write them less after replacing the amplifier
Check the supply for noise and the connectors for shorts

AL.16: Encoder initial communication error 1 (MR-J4) / Encoder error 1 (MR-J2S, MR-J3)

Hits OEE: Availability

What it means: The amplifier could not communicate with the servo motor encoder when it was switched on.^[1]^[2]^[3]

Also on: Same code on all three; MR-J3 calls it Encoder error 1 (At power on); MR-J4 renames it and adds detail numbers AL.16.1 to AL.16.F.

How it clears: Power off and on only

Likely causes

Encoder connector CN2 loose or disconnected
Encoder cable broken or shorted
Encoder faulty, or a motor from another series connected (MR-J3)
Noise: encoder cable run beside power cables, poor grounding or shield not connected; on MR-J3 also the wrong 2-wire or 4-wire cable setting in PC22

First checks, in this order

With power off, reseat CN2 at the amplifier and the encoder connector at the motor
Check the encoder cable for breaks or shorts and confirm the shield is connected
Check that the encoder cable is not bundled with power cables and that amplifier and motor are grounded
Swap in a known good motor or amplifier to see which part the alarm follows

AL.17: Board error

Hits OEE: Availability

What it means: A CPU or other part on the amplifier board, such as the current detection circuit, is faulty.^[1]^[2]^[3]

Also on: Same number and name on MR-J2S, MR-J3 and MR-J4 (AL.17.1 to AL.17.9).

How it clears: Power off and on only

Likely causes

Faulty parts in the amplifier
MR-J2S: amplifier outputs U, V, W not connected to the motor
Noise or high ambient temperature around the amplifier (MR-J4)

First checks, in this order

Disconnect every cable except the control circuit power and switch on: if AL.17 comes back, replace the amplifier
On MR-J2S, check that U, V and W of the amplifier are wired to U, V and W of the motor
On MR-J4, if it appears only at servo-on, the current detection circuit is suspect and the amplifier should be replaced

AL.19: Memory error 3 (Flash-ROM)

Hits OEE: Availability

What it means: The ROM (Flash-ROM) memory in the amplifier has failed.^[1]^[2]^[3]

Also on: MR-J2S calls it Memory error 3 (ROM memory fault); same code on MR-J3 and MR-J4 (AL.19.1 to AL.19.3).

How it clears: Power off and on only

Likely causes

Faulty ROM in the amplifier
Noise or temperature problem near the amplifier (MR-J4)

First checks, in this order

Disconnect every cable except the control circuit power and switch on: if AL.19 comes back, replace the amplifier
If it does not come back, check for noise and ambient temperature, then reconnect cables one at a time

AL.1A: Servo motor combination error (MR-J4) / Motor combination error (MR-J2S, MR-J3)

Hits OEE: Availability

What it means: The connected servo motor does not match the amplifier.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.1A.1, AL.1A.2 and AL.1A.4.

How it clears: Power off and on only

Likely causes

Wrong motor model connected to this amplifier, often after a motor or amplifier was swapped
MR-J4: Pr. PA01 motor type setting (rotary, linear or direct drive) does not match the connected motor

First checks, in this order

Read the motor and amplifier nameplates and check the combination against the manual's combination table
On MR-J4, check the Pr. PA01 setting for the motor type in use
Fit the correct motor or amplifier and power cycle

AL.20: Encoder normal communication error 1 (MR-J4) / Encoder error 2 (MR-J2S, MR-J3)

Hits OEE: Availability

What it means: Communication between the encoder and the amplifier failed while the machine was running.^[1]^[2]^[3]

Also on: Same code on all three; MR-J3 calls it Encoder error 2 (during runtime); MR-J4 renames it and adds detail numbers AL.20.1 to AL.20.A.

How it clears: Power off and on only

Likely causes

Encoder cable or CN2 connector disconnected, broken or shorted
Encoder cable shield not connected to the connector ground plate (MR-J4)
Motor oscillating so hard that the encoder detects excessive acceleration
Encoder faulty or external noise

First checks, in this order

Check CN2 and the encoder cable for breaks, shorts and damage where it flexes
Check the shield connection and keep the encoder cable away from power cables
If the motor vibrates or makes unusual noise, lower the position loop gain or the auto tuning response
If it repeats with a good cable, replace the servo motor

AL.21: Encoder normal communication error 2 (MR-J4) / Encoder error 3 (MR-J3)

Hits OEE: Availability

What it means: The encoder itself detected an error signal during operation.^[1]^[2]

Also on: MR-J3 and MR-J4 only; not in the MR-J2S-A alarm list.

How it clears: Power off and on only

Likely causes

Detection circuit fault in the encoder
Encoder saw a very high speed or acceleration because of oscillation (MR-J4)
Encoder cable shield not connected, or noise

First checks, in this order

Lower the loop gain and check whether the alarm repeats
Check the encoder cable shield and grounding
If it keeps repeating, replace the servo motor

AL.24: Main circuit error

Hits OEE: Availability

What it means: A ground fault or short was detected on the motor power output U, V, W.^[1]^[2]^[3]

Also on: Same number and meaning on all three, but the reset rule differs by generation (see reset).

How it clears: MR-J2S: power off and on only. MR-J3-A: RES, SET or power cycle. MR-J4: AL.24.1 (hardware detection) power cycle only, AL.24.2 (software detection) RES, SET or power cycle.

Likely causes

Motor power cable insulation damaged, causing a short or ground fault
Ground fault inside the servo motor
Input power wires touching the motor power wires at the main circuit terminal block
Amplifier main circuit fault, or on MR-J3 a faulty external dynamic brake

First checks, in this order

With power off and the charge lamp out, disconnect U, V, W at the amplifier and switch servo-on: if AL.24 still occurs, replace the amplifier
Disconnect the cable at the motor end and check the insulation of the motor and of the cable between U, V, W and ground
Check that the main circuit input wires and the motor wires are not touching
On MR-J4, check the regenerative resistor wiring and that the resistor matches the amplifier

AL.25: Absolute position erased (MR-J4) / Absolute position erase (MR-J2S, MR-J3)

Hits OEE: Availability

What it means: The absolute position data is lost, or the absolute system was powered on for the first time.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.25.1 (motor encoder) and AL.25.2 (scale measurement encoder).

How it clears: Power off and on only, then home position setting is mandatory

Likely causes

First power on after setting up the absolute position detection system
Battery low, disconnected or faulty, or the battery removed while the control power was off
Encoder cable fault or voltage drop in a long or non-recommended encoder cable
Encoder fault, or on J2S the encoder super capacitor not yet charged

First checks, in this order

Check the battery is fitted and connected, and measure its voltage
Check the encoder cable and battery cable connections
Leave the alarm on for a few minutes, then switch power off and on (J2S and J3 instruction)
Always perform home position setting again before running

AL.30: Regenerative error

Hits OEE: Availability

What it means: The regenerative power exceeded what the built-in regenerative resistor or regenerative option can absorb, or the regenerative transistor is faulty.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.30.1 (heat error), AL.30.2 (signal error) and AL.30.3 (feedback signal error).

How it clears: RES, SET or power cycle, but only after removing the cause and about 30 minutes of cooling; do not power cycle repeatedly

Likely causes

Regenerative option setting wrong (Pr. PA02 on J3 and J4, parameter No.0 on J2S) or the resistor not connected
Frequent positioning or continuous regeneration, such as short deceleration times or a heavy load
Supply voltage too high (J4: over 264 V AC on 200 V class; J2S: 260 V AC or more)
Regenerative resistor, option or transistor faulty

First checks, in this order

Read the regenerative load ratio on the status display or MR Configurator (100% or more on J4 means it was exceeded)
Check that PA02 (No.0 on J2S) matches the resistor or option actually fitted and that it is wired correctly
Measure the input voltage
Reduce positioning frequency, lengthen the deceleration time, reduce the load or fit a larger regenerative option; if the alarm appears even with the resistor removed, replace the amplifier

AL.31: Overspeed

Hits OEE: Availability

What it means: The servo motor speed exceeded the instantaneous permissible speed.^[1]^[2]^[3]

Also on: Same number and name on all three; MR-J4 detail AL.31.1 Abnormal motor speed.

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Command pulse frequency too high
Acceleration or deceleration time too short, causing a large overshoot
Unstable servo gains causing overshoot
Electronic gear ratio too large, wrong U, V, W wiring (J4) or a faulty encoder

First checks, in this order

Check the command pulse frequency from the controller
Check the electronic gear (PA06 and PA07 on J3 and J4, parameters No.3 and No.4 on J2S)
Lengthen the acceleration and deceleration time and check for oscillation; retune the gains if needed
Check the U, V, W wiring to the motor; on MR-J4 network types (B, GF) also check the controller's speed command and the overspeed alarm level in Pr. PC08

AL.32: Overcurrent

Hits OEE: Availability

What it means: A current higher than the amplifier's permissible current flowed.^[1]^[2]^[3]

Also on: Same number and meaning on all three, but the reset rule differs by generation (see reset).

How it clears: MR-J2S: RES, SET or power cycle. MR-J3-A: power off and on only. MR-J4: AL.32.1 and AL.32.3 (hardware detection) power cycle only, AL.32.2 and AL.32.4 (software detection) RES, SET or power cycle.

Likely causes

Short or ground fault on the motor power cable or in the motor
Faulty power transistor (IPM) in the amplifier
Dynamic brake or external dynamic brake fault, or wrong regenerative resistor wiring
External noise making the overcurrent detection circuit trip

First checks, in this order

With power off, disconnect U, V, W at the amplifier and switch servo-on: if AL.32 still occurs, the amplifier is faulty
Check the motor cable and the motor insulation between U, V, W and ground
Check the regenerative resistor wiring and the grounding and noise sources near the amplifier
If AL.32 returns right after a reset and servo-on, stop power cycling: the manual warns the power transistor may be damaged

AL.33: Overvoltage

Hits OEE: Availability

What it means: The DC bus voltage exceeded its limit: 400 V DC on 200 V class and 100 V class models, 800 V DC on 400 V class.^[1]^[2]^[3]

Also on: Same number and meaning on all three, but MR-J2S needs a power cycle to clear it.

How it clears: MR-J2S: power off and on only. MR-J3-A and MR-J4: RES, SET or power cycle.

Likely causes

No regenerative option where one is needed, or PA02 (No.0 on J2S) set to 'not used' although one is fitted
Regenerative resistor lead open or broken
Regenerative capacity too small for the deceleration, or supply voltage too high
Regenerative transistor fault, ground fault on U, V, W, or the BUE to SD jumper of an FR-BU2 brake unit removed

First checks, in this order

Check the regenerative option setting and its wiring
Measure the resistance of the built-in resistor or regenerative option
Set a longer deceleration time and see whether the alarm stops; if it does, fit a larger regenerative option
Measure the input voltage (J4 limit 264 V AC on 200 V class)

AL.34: SSCNET receive error 1

Hits OEE: Availability

What it means: MR-J4-B type: continuous SSCNET III/H communication errors with the controller.^[1]

Also on: SSCNET (B type) amplifiers only, so it is not in the MR-J2S-A or MR-J3-A lists; the MR-J2S-B and MR-J3-B manuals were not checked.

How it clears: Controller reset command, CPU reset or power cycle (MR-J4-B)

Likely causes

SSCNET III cable disconnected
Dirty cable end, or a broken or severed cable
Cable wrapped in vinyl tape or touching wire insulation that contains migrating plasticizer
Faulty amplifier

First checks, in this order

Switch off the amplifier's control circuit power, then reconnect the SSCNET III cable
Wipe the dirt off the cable tip and check whether the alarm repeats
Check the cable for damage and for contact with taped or PVC cables, and replace it if needed
If it still repeats, replace the amplifier

AL.35: Command frequency error (MR-J4) / Command pulse frequency error (MR-J2S, MR-J3)

Hits OEE: Availability

What it means: The frequency of the input command pulse is too high.^[1]^[2]^[3]

Also on: Same code and meaning on all three; the MR-J3-A alarm list calls it Command pulse frequency alarm.

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Command pulse frequency above the amplifier's limit
Noise entering the command pulse wiring
Faulty command device, or on MR-J4 the command input pulse train filter in Pr. PA13 set wrong
Manual pulse generator turned too fast (MR-J4)

First checks, in this order

Compare the controller's maximum output pulse frequency with the amplifier input specification and the PA13 filter setting
Check the shielding of the pulse wiring and run it away from power cables; the J2S manual recommends a differential line driver instead of open collector
Swap the command device to rule it out

AL.37: Parameter error

Hits OEE: Availability

What it means: A parameter setting is wrong or has been corrupted.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.37.1 range error, AL.37.2 combination error and AL.37.3 point table error, and does not record AL.37 in the alarm history.

How it clears: Power off and on (J2S, J3, J4-A); on MR-J4-B, AL.37.1 and AL.37.2 also clear with a CPU reset

Likely causes

A parameter out of range or contradicting another parameter
A regenerative option selected in PA02 (No.0 on J2S) that cannot be used with this amplifier
EEP-ROM written more than 100,000 times, or an amplifier fault that rewrote parameters
MR-J2S: alarm code output (parameter No.49) set together with the absolute position system or with MBR on pin CN1B-19

First checks, in this order

Read the parameter error number in alarm mode on the display, which shows which parameter is faulty, and correct it
Check PA02 (No.0 on J2S) against the regenerative option actually fitted
If parameters keep changing by themselves, replace the amplifier

AL.45: Main circuit device overheat

Hits OEE: Availability

What it means: The power devices inside the amplifier overheated.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.45.1 and AL.45.2.

How it clears: RES, SET or power cycle, but only after about 30 minutes of cooling

Likely causes

Ambient temperature above 55°C (131°F), or close mounting outside the specification
Power switched on and off repeatedly while overloaded
Cooling fan stopped, or fan, heat sink or openings clogged with dirt
Faulty amplifier

First checks, in this order

Measure the temperature inside the cabinet next to the amplifier
Check the mounting clearances against the manual
Clean the cooling fan, heat sink and openings and check the fan runs
Stop using power cycles to clear overload alarms

AL.46: Servo motor overheat

Hits OEE: Availability

What it means: The servo motor's thermal sensor tripped because the motor got too hot.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.46.1 to AL.46.6.

How it clears: RES, SET or power cycle, but only after about 30 minutes of cooling

Likely causes

Motor ambient temperature above 40°C (104°F)
Motor overloaded
Faulty thermal sensor in the encoder
MR-J4: thermistor wire problem on motors with thermistors (AL.46.3)

First checks, in this order

Check the motor surroundings and ventilation
Check the effective load ratio on the status display or MR Configurator and reduce the load or change the operation pattern
If the motor is cool and the load ratio is low, the thermal sensor is faulty: replace the motor

AL.47: Cooling fan error (MR-J4) / Cooling fan alarm (MR-J3)

Hits OEE: Availability

What it means: The amplifier's cooling fan stopped or slowed below the alarm level.^[1]^[2]

Also on: MR-J3 and MR-J4 only (J4 details AL.47.1 fan stop, AL.47.2 speed reduction); on MR-J2S a stopped fan shows up as a cause of AL.45.

How it clears: Power off and on only

Likely causes

Cooling fan at the end of its life
Foreign matter caught in the fan
Fan power supply failure (MR-J3)

First checks, in this order

Look for and remove anything caught in the fan
Check whether the fan turns
Replace the fan (J3) or the amplifier (J4)

AL.50: Overload 1

Hits OEE: Availability

What it means: The load exceeded the amplifier's overload protection characteristic, usually a long overload.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.50.1 to AL.50.6 (during operation and during a stop).

How it clears: RES, SET or power cycle, but only after about 30 minutes of cooling; do not power cycle repeatedly

Likely causes

Running above the amplifier's continuous output current, or machine friction or binding
Electromagnetic brake not released
Servo system unstable and hunting
U, V, W wired to the wrong motor phases, power cable disconnected, or encoder fault

First checks, in this order

Check the effective load ratio on the status display or MR Configurator
Check that the brake releases and that the machine turns freely
Check U, V, W order and the motor power cable
If the axis hunts, rerun auto tuning or lower the response; then reduce the load or fit a larger motor

AL.51: Overload 2

Hits OEE: Availability

What it means: Maximum output current flowed continuously for a few seconds, usually because the machine collided with something.^[1]^[2]^[3]

Also on: Same on all three; MR-J4 adds AL.51.1 (during operation) and AL.51.2 (during a stop).

How it clears: RES, SET or power cycle, but only after about 30 minutes of cooling

Likely causes

Machine struck something or is jammed
U, V, W wired to the wrong motor phases, or encoder cable wrong
Servo system unstable and hunting
Brake applied, power cable disconnected, encoder or motor fault

First checks, in this order

Look for a crash or jam and check limit switches
Check U, V, W order and the encoder cable connection
On MR-J4, check the peak load ratio for torque saturation
Retune if the axis oscillates, then reduce the load or fit a larger motor

AL.52: Error excessive

Hits OEE: Availability

What it means: The position error between the command and the actual motor position exceeded the alarm level (2.5 revolutions on J2S software B0 or later, 3 revolutions on J3, set by Pr. PC24 and PC43 on J4-A).^[1]^[2]^[3]

Also on: Same code and meaning on all three; the alarm threshold differs by generation as stated.

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Acceleration or deceleration time too short, or the torque limit too low
Not enough torque because of low supply voltage or an undersized motor
Motor moved by an external force, a collision or a brake that did not release
Position loop gain too low, U, V, W miswired, broken power cable or encoder fault

First checks, in this order

Check the brake releases and the machine is not jammed
Check the torque limit (PA11 and PA12 on J3, No.28 on J2S) and the bus voltage
Lengthen the acceleration and deceleration time
Check U, V, W wiring and raise the position loop gain (PB08 on J3 and J4) if it is low

AL.8A: Serial communication time-out error (MR-J2S, MR-J3) / USB communication time-out error, serial communication time-out error, Modbus RTU communication time-out error (MR-J4)

Hits OEE: Availability

What it means: Communication with the PC or controller over RS-232C, RS-422, USB or Modbus RTU stopped for longer than the set time.^[1]^[2]^[3]

Also on: Same code on all three; the name grows with the interfaces supported.

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Communication cable broken or disconnected
Communication cycle longer than the time-out setting (parameter No.56 on J2S)
Wrong protocol, or the PC or controller stopped sending
Faulty amplifier (MR-J4)

First checks, in this order

Check the communication cable and replace it if in doubt
Check that the PC or controller is still sending commands
Check the time-out setting and the protocol settings

AL.8E: Serial communication error (MR-J2S, MR-J3) / USB communication error, serial communication error, Modbus RTU communication error (MR-J4)

Hits OEE: Availability

What it means: A communication error occurred between the amplifier and a PC or controller.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.8E.1 to AL.8E.8.

How it clears: RES on, SET on the current alarm display, or power off and on (MR-J4-B: controller reset, CPU reset or power cycle)

Likely causes

Communication cable open or shorted
Faulty PC or communication device, or wrong settings on it
Wrong character code, command or data number (MR-J3)

First checks, in this order

Check the communication settings of the PC or device
Replace the communication cable and check whether the alarm repeats
If it still repeats, replace the amplifier (MR-J4)

88888: Watchdog

Hits OEE: Availability

What it means: The CPU or another part in the amplifier failed.^[1]^[2]^[3]

Also on: Same on all three; the MR-J4 manual lists the detail as 88._ or 8888._ and also writes it as AL. 888.

How it clears: Power off and on only

Likely causes

Faulty parts in the amplifier
CPU upset by external noise (MR-J3)

First checks, in this order

Note that 88888 flashing for an instant at power on is normal, not an error (MR-J3)
Disconnect every cable except the control circuit power and switch on: if 88888 stays, replace the amplifier
Check grounding and nearby noise sources such as valves, contactors and relays

AL.92: Battery cable disconnection warning (MR-J3, MR-J4) / Open battery cable warning (MR-J2S)

Warning: act before it becomes a stop

What it means: The battery voltage reaching the encoder for the absolute position system is low, or the battery cable is open.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.92.1 and AL.92.3 (battery degradation).

How it clears: Clears automatically once the cause is removed

Likely causes

Battery not connected or battery cable open
Battery voltage at the encoder low (about 3.2 V on J2S, about 3 V on J3, below 3.1 V on J4)
Encoder cable broken

First checks, in this order

Check the battery connection (CN4 on MR-J4) and the battery cable
Measure the battery voltage with a tester and replace the battery if low
Check the encoder cable for a break or short

AL.96: Home position setting warning

Warning: act before it becomes a stop

What it means: Home position setting could not be made.^[1]^[2]^[3]

Also on: Same code on all three; the MR-J3-A alarm list calls it Home position setting error; MR-J4 adds AL.96.1 to AL.96.4.

How it clears: Clears automatically once the cause is removed

Likely causes

Droop pulses still larger than the in-position range when home was set
Command pulses entered during home position setting
Creep speed too high, or on MR-J4 home set while servo-off

First checks, in this order

Make sure the axis has settled within the in-position range before setting home
Do not send command pulses during home position setting
Reduce the creep speed and set home again

AL.99: Stroke limit warning

Warning: act before it becomes a stop

What it means: The stroke end signal (LSP or LSN) for the commanded direction is off.^[1]^[2]

Also on: MR-J3-A and MR-J4 (A and GF types); not in the MR-J2S-A list.

How it clears: Clears automatically once the cause is removed

Likely causes

Axis reached the forward (LSP) or reverse (LSN) stroke end limit switch
Limit switch not wired to LSP or LSN, or the input not turned on

First checks, in this order

Check the limit switch wiring and the LSP and LSN state on the external I/O signal display
Review the moving range so the axis does not hit the stroke ends

AL.9F: Battery warning

Warning: act before it becomes a stop

What it means: The amplifier detected that the absolute position battery voltage is low.^[1]^[2]^[3]

Also on: Same code on all three; thresholds differ because the battery types differ.

How it clears: Clears automatically once the cause is removed

Likely causes

Battery voltage low (3.2 V or less on J2S and J3, below 4.9 V DC on the MR-J4 battery)
Battery not connected to CN4 (MR-J4)
Encoder cable problem (MR-J4)

First checks, in this order

Measure the battery voltage and replace the battery
Check the battery is connected correctly
Replace the battery with the control power on, because removing it with control power off causes AL.25 on MR-J4

AL.E0: Excessive regeneration warning (MR-J3, MR-J4) / Excessive regenerative warning (MR-J2S)

Warning: act before it becomes a stop

What it means: Regenerative power reached 85% of the permissible level, so AL.30 may follow.^[1]^[2]^[3]

Also on: Same code and meaning on all three.

How it clears: Clears automatically; if power is cycled during it, allow more than 30 minutes of cooling before running

Likely causes

Frequent positioning or short deceleration times
Heavy load or regenerative option too small

First checks, in this order

Check the regenerative load ratio on the status display or MR Configurator
Reduce positioning frequency, lengthen deceleration or reduce the load
Fit a larger regenerative option

AL.E1: Overload warning 1 (MR-J3, MR-J4) / Overload warning (MR-J2S)

Warning: act before it becomes a stop

What it means: The load reached 85% of the AL.50 or AL.51 alarm level.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.E1.1 to AL.E1.8.

How it clears: Clears automatically; if power is cycled during it, allow more than 30 minutes of cooling before running

Likely causes

High effective load: running above the continuous rating, machine friction, or a brake that does not release
Hunting from unstable gains, or wrong U, V, W wiring (the same causes as AL.50 and AL.51)

First checks, in this order

Check the effective load ratio
Work through the AL.50 and AL.51 checks before the alarm trips

AL.E3: Absolute position counter warning

Warning: act before it becomes a stop

What it means: The absolute encoder pulses are faulty, or the multi-revolution counter went past its maximum range.^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.E3.1, AL.E3.2, AL.E3.4 and AL.E3.5.

How it clears: Clears automatically once the cause is removed, but home position setting is mandatory

Likely causes

Noise entering the encoder
Encoder fault
Axis travelled more than about 32,767 revolutions from home in one direction

First checks, in this order

Check the encoder cable shielding, grounding and noise sources
Check the machine's travel range from home
Always perform home position setting again after removing the cause

AL.E5: ABS time-out warning

Warning: act before it becomes a stop

What it means: The absolute position data transfer with the PLC timed out (on MR-J4, no PLC response within 5 s, or ABSM, SON, RES or the forced stop turned off during the transfer).^[1]^[2]^[3]

Also on: General-purpose interface (A) types on all three.

How it clears: Clears automatically once the cause is removed

Likely causes

PLC ladder program wrong
Wiring of the ABS transfer signals wrong, such as ST2 and TLC

First checks, in this order

Check the I/O wiring for loose or broken wires
Check the ABS transfer sequence in the PLC program

AL.E6: Servo forced stop warning (MR-J4) / Servo emergency stop warning (MR-J2S, MR-J3)

Warning: act before it becomes a stop

What it means: The forced stop input is off (EMG on J2S and J3, EM2 or EM1 on J4), so the amplifier is in servo-off.^[1]^[2]^[3]

Also on: Same code on all three; the name and input signal changed on MR-J4.

How it clears: Clears automatically when the forced stop input turns back on

Likely causes

Emergency stop pressed or the emergency stop circuit open
External 24 V DC power supply off (MR-J4)
SS1 command from the functional safety unit (MR-J4)

First checks, in this order

Make the machine safe, then release the emergency stop
Check the emergency stop circuit wiring and the 24 V DC supply to the input
Check the input state on the external I/O signal display

AL.E7: Controller forced stop warning

Warning: act before it becomes a stop

What it means: The servo system controller's forced stop signal is active.^[1]

Also on: MR-J4 (mainly B types); not in the MR-J2S-A or MR-J3-A lists, and the J2S-B and J3-B manuals were not checked.

How it clears: Clears automatically once the forced stop is cancelled

Likely causes

Forced stop input of the controller is on
On MR-J4-A, a forced stop sent from the controller over Modbus RTU

First checks, in this order

Check the controller's forced stop state and its input wiring
Make the machine safe, then cancel the controller's forced stop

AL.E8: Cooling fan speed reduction warning

Warning: act before it becomes a stop

What it means: The amplifier's cooling fan speed dropped to the warning level.^[1]^[2]

Also on: MR-J3 and MR-J4 only; MR-J3 notes it is not shown on MR-J3-70A and MR-J3-100A; not in the MR-J2S-A list.

How it clears: Clears automatically once the cause is removed

Likely causes

Cooling fan near the end of its life
Foreign matter caught in the fan
Fan power supply broken (MR-J3)

First checks, in this order

Remove anything caught in the fan
Check the amplifier's total power-on time against the fan life (MR-J4)
Replace the fan (J3) or the amplifier (J4) before it becomes AL.47

AL.E9: Main circuit off warning

Warning: act before it becomes a stop

What it means: Servo-on was turned on while the main circuit power was off (on MR-J4 also a bus voltage drop while running under 50 r/min).^[1]^[2]^[3]

Also on: Same code on all three; MR-J4 adds AL.E9.1 to AL.E9.4.

How it clears: Clears automatically once the cause is removed

Likely causes

Main circuit power not switched on, for example the magnetic contactor did not close
Main circuit power wiring disconnected, or on MR-J4 the P3 to P4 connection open
MR-J4: Pr. PA02 magnetic contactor drive output setting does not match the wiring, or the bus voltage is low

First checks, in this order

Check that the main circuit supply is on and the contactor closes
Check the main circuit wiring and, on MR-J4, the P3 to P4 connection
On MR-J4, check the PA02 setting and the supply capacity

AL.EA: ABS servo-on warning

Warning: act before it becomes a stop

What it means: Servo-on was not given within 1 s after the amplifier entered absolute position data transfer mode, so it stays servo-off.^[1]^[2]^[3]

Also on: General-purpose interface (A) types on all three.

How it clears: Clears automatically once the cause is removed

Likely causes

PLC ladder program wrong
Servo-on (SON) wiring wrong

First checks, in this order

Check the SON wiring
Check the timing of SON in the ABS transfer sequence of the PLC program

AL.EC: Overload warning 2

Warning: act before it becomes a stop

What it means: Operation above the rated output was repeated while the motor shaft was not turning (MR-J3 adds that the current is concentrated in one of the U, V, W phases).^[1]^[2]

Also on: MR-J3 and MR-J4 only; not in the MR-J2S-A list.

How it clears: Clears automatically; on MR-J4, if power is cycled during it, allow more than 30 minutes of cooling

Likely causes

Holding or pushing a heavy load at standstill
High positioning frequency or a motor too small for the load

First checks, in this order

Reduce the positioning frequency
Reduce the load, or fit a larger capacity motor and amplifier

AL.ED: Output watt excess warning

Warning: act before it becomes a stop

What it means: The motor's output (speed times torque) stayed above the rated output: above 150% on MR-J3, above 120% on MR-J4.^[1]^[2]

Also on: MR-J3 and MR-J4 only, with different thresholds; not in the MR-J2S-A list.

How it clears: Clears automatically once the cause is removed

Likely causes

Continuous running at high speed and high torque
Load too large for the motor

First checks, in this order

Check motor speed and torque on the monitor
Reduce the speed or the load, or fit a larger capacity motor and amplifier

Log it, or the same alarm comes back

An alarm stops the servo axis, and with it the machine, so in OEE terms it is a breakdown. A warning does not stop it yet. Log alarms with the minutes lost and the axis, and count warnings that repeat before they become stops.

Reason code: the code plus the axis, for example MR-J4 AL.50, press 2 feed axis, so an overload that trips every Monday after a cold start stands out.
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.

For Mitsubishi CNC controls, fabrico.io has the Mitsubishi CNC alarm codes guide. An alarm code tells you what the servo drive 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 axis stopped next to the alarm.

Frequently asked questions

How do I reset an alarm on a Mitsubishi MR-J4, MR-J3 or MR-J2S servo amplifier?

Remove the cause and make sure servo-on is off so the axis cannot restart suddenly, then turn on RES (Reset), press SET on the current alarm display, or switch the power off and on; MR-J4-A can also reset from the MR Configurator2 Alarm Display window, and MR-J4-B is reset by the controller's reset command, a CPU reset or a power cycle. Each alarm has its own allowed methods, so encoder, memory and board alarms clear only with a power cycle. Warnings clear by themselves once the cause is gone.

Why does AL.16 or AL.20 come back after I press reset?

On all three generations the encoder alarms (AL.16, AL.20, and AL.21 on J3 and J4), the memory and board alarms AL.12 to AL.19, AL.1A, AL.25 and the 88888 watchdog cannot be cleared with RES or SET, only by switching power off and on. If the encoder cable, the CN2 connector or the encoder is still faulty, the alarm returns at the next power on, so check the connector, cable and shield first.

Do the alarm codes mean the same on MR-J2S, MR-J3 and MR-J4?

The main numbers carry over, so AL.10 is undervoltage, AL.32 overcurrent and AL.50 overload 1 on all three. MR-J4 adds a detail number after the dot and renames some codes, for example AL.16 Encoder error 1 became Encoder initial communication error 1 and AL.E6 Servo emergency stop warning became Servo forced stop warning. A few reset rules also differ: AL.32 clears with RES on J2S but needs a power cycle on J3, and AL.33 is the opposite.

How do I see and clear the alarm history on a MELSERVO amplifier?

In alarm mode the display shows the current alarm and the last six alarms (press UP or DOWN to step through them on J2S and J3), and MR Configurator or MR Configurator2 shows the history with alarm details. To clear it, set the alarm history clear parameter (No.16 on J2S, PC18 on J3 and J4-A, PC21 on J4-B) to 1 and switch power off and on; the setting returns to 0 by itself. MR-J4 does not record AL.37 or warnings in the history.

Why must I wait 30 minutes after AL.30, AL.50 or AL.51 before restarting?

These are thermal protections, and the manuals warn that clearing them and restarting again and again, for example by power cycling, can damage the amplifier and the motor. Remove the cause and allow about 30 minutes of cooling before resuming; the same rule applies to AL.45 and AL.46, and to warnings AL.E0 and AL.E1 if the power is cycled while they are showing.

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.

See the moment the axis stopped

Fabrico, a light MES with computer vision, syncs camera footage with machine and timeline data, so you can replay the exact moment the axis stopped and review the cause next to its alarm. 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.

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