Metal Detector Troubleshooting: Causes & Fixes

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Problems on this page:

False rejects Missed contamination or low sensitivity Reject mechanism fault Test pieces not detected at the routine check Settings left on the wrong product Metal near the aperture causing signals Rejected packs not kept apart securely Detector unstable after washdown

False rejects

Hits OEE: Performance

Symptoms: Good product rejected, high reject rate, yield loss.

Product effect not compensated

Set up product learn/phasing; use the correct memory.

Electrical noise or vibration

Improve grounding, shielding and mounting; reduce vibration.

Wet/foil product handling

Use the right frequency/mode for the product.

Prevention: Proper product setup, EMC/mounting, correct mode.

Missed contamination or low sensitivity

Hits OEE: Quality

Symptoms: Failed test-piece checks, contamination passes, safety risk.

Sensitivity set too low or wrong setup

Set sensitivity to the smallest reliable sphere; re-learn product.

Aperture too large for the pack

Match aperture to product size; centre the product.

Drift or head fault

Run routine test-piece checks; service the head.

Prevention: Correct sensitivity, right aperture, routine validation.

Reject mechanism fault

Hits OEE: Availability

Symptoms: Reject does not fire or misses, contaminated product passes, stops.

Air supply or actuator fault

Check air pressure and the reject actuator/valve.

Timing or sensor issue

Set reject timing; check the product-tracking sensor.

Bin-full or confirm-sensor fault

Empty the reject bin; check confirmation sensors.

Prevention: Air/actuator maintenance, reject-timing checks, fail-safe interlocks.

Test pieces not detected at the routine check

Hits OEE: Quality

Symptoms: A test piece passes without a reject during the start-of-shift or hourly check.

Test piece passed at an easy position

Pass test pieces at the least sensitive point of the aperture, as your procedure says.

Wrong test piece for the product spec

Use the test piece sizes and metals your specification names.

Settings changed since the last check

Check the product setting and sensitivity before testing.

Prevention: Test procedure written, followed and recorded.

Settings left on the wrong product

Hits OEE: Quality

Symptoms: Rejects rise or test pieces pass straight after a product change.

Product setting not changed at changeover

Put the metal detector setting on the changeover checklist.

Product learned badly

Relearn the product as the maker describes.

Settings open to anyone

Restrict who can change detector settings.

Prevention: Detector check signed off at every changeover.

Metal near the aperture causing signals

Hits OEE: Quality

Symptoms: Unexplained rejects that started after a change to the conveyor or guarding.

Metal guards or rollers moved into the metal-free zone

Restore the metal-free zone the maker specifies.

Moving metal parts nearby

Move or isolate moving metal parts near the head.

Loose metal fixings vibrating

Tighten and secure fixings near the detector.

Prevention: Any change near the detector checked against the maker's installation rules.

Rejected packs not kept apart securely

Hits OEE: Quality

Symptoms: Rejected packs found back in the line or handled without a record.

Reject bin not lockable

Use a lockable reject bin.

No procedure for rejected packs

Write who opens the bin, how rejects are checked and how they are recorded.

Bin full or confirmation missing

Fit bin-full and reject confirmation sensors.

Prevention: Reject handling procedure followed and recorded.

Detector unstable after washdown

Hits OEE: Quality

Symptoms: False rejects or drift after cleaning, often settling later in the shift.

Water ingress in the head

Check the head seals and dry the detector.

Condensation

Allow the detector to stabilise before production.

Product needs relearning after cleaning

Relearn the product if the maker recommends it.

Prevention: A restart test after every washdown.

Guidance only. Always follow lockout/tagout and your site's safe-work procedures, and verify against OEM manuals before acting.

Stop the same fault coming back

Recurring metal detector stops usually trace to a cause you cannot see by hand. The partner we recommend is Fabrico: it tracks OEE and downtime live from PLCs, IoT sensors or AI cameras; where cameras are fitted, the video of a stop sits next to its machine data so the cause can be reviewed; and a technician opens the work order from the stop in the same system. Data is hosted in the AWS EU region, and Fabrico and the company behind it hold ISO/IEC 27001, ISO/IEC 20000-1 and ISO 9001.

Book a Fabrico demo

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Methods that cut recurring stops: the six big losses · root cause analysis · preventive vs predictive maintenance · TPM · SMED & changeover