ApprenticeMedium risk

Limit switch or proximity sensor not being detected

A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.

Start fault-finding

Safety first

Defeating or jumpering a limit/safety switch can let machinery overtravel and crush. Never bypass a safety-related switch to 'test' — confirm its purpose first.

Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
Mechanical / alignment problem
Most likely
The target has drifted out of range, the actuator lever is bent, the sensing gap is too large, or the photo-eye/reflector is misaligned or dirty.
2
Sensor not powered or wrong wiring
#2
Missing supply to a powered sensor, or a sourcing/sinking (PNP/NPN) mismatch with the input, leaves it unable to drive the input correctly.
3
Damaged sensor or cable
#3
A crushed cable, water ingress, or a failed sensor head means it can't switch even when the target is present.
4
Wrong sensor type for the target
#4
An inductive prox aimed at a non-metallic target, or a sensor with too little range for the gap, simply won't detect.
5
Input card / channel fault
Least likely
The sensor switches correctly but the controller input channel doesn't read it.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 4

Present the target/operate the switch and watch the sensor's own indicator LED (if fitted).

Expected reading

The sensor LED changes state cleanly as the target enters/leaves range.

Your actual reading

If it passes

The sensor is detecting — the problem is between the sensor and the input. Check wiring/input.

If it fails

The sensor isn't detecting — check alignment/gap, power, and sensor type.

Often leads toPLC output LED is on but the device doesn't work Motor goes one way but won't go the other (e.g. down but not up)

View all expected readings at once

1. Present the target/operate the switch and watch the sensor's own indicator LED (if fitted).

The sensor LED changes state cleanly as the target enters/leaves range.

2. Check alignment, sensing gap, and cleanliness; confirm the sensor has its supply.

Target within rated range, faces clean, supply present at the sensor.

3. If the sensor LED switches, measure the sensor's output signal at the input terminal as the target moves.

The output voltage/state changes at the input terminal in step with the target.

4. If the signal reaches the input but isn't read, test the input on a known-good channel or with a known input.

A known input toggles the controller's input bit.

Fault-finding flowchart

The same logic as a decision tree.

1
start
Sensor not detected
→ step 2
2
decision
Does the sensor's own LED switch with the target?
Yes→ step 3No→ step 4
3
decision
Does the signal reach the controller input terminal?
Yes→ step 5No→ step 6
4
decision
Are alignment, gap, cleanliness and supply all correct?
Yes→ step 7No→ step 8
5
result
Signal arrives but isn't read — suspect the input channel/configuration.
6
result
Broken cable, PNP/NPN mismatch, or loose terminal between sensor and input.
7
result
Suspect a damaged sensor/cable or wrong sensor type for the target.
8
result
Fix alignment/gap/clean faces/restore supply, then re-test.

Common mistakes apprentices make

Jumping straight to 'the sensor's dead' without checking the gap and alignment first.
Mixing up PNP and NPN sensors with the input type so the signal never registers.
Aiming an inductive prox at a non-metallic target.
Bypassing a limit switch to test — dangerous on anything that can overtravel.

When to stop & escalate

Any sensor that forms part of a safety function (guard, overtravel, slack-rope) must be confirmed against the safety design and never defeated. If an input channel has failed, follow site procedure for controller maintenance and program backups.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

PLC inputs / outputs

PLC output LED is on but the device doesn't work

The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.

5 causes4 test steps

QualifiedMedium risk

Forward / reverse circuits

Motor goes one way but won't go the other (e.g. down but not up)

A reversing drive works in one direction only. One command (say, down) runs fine; the other (up) does nothing, or just hums/trips. Common on hoists, doors, and conveyors.

5 causes4 test steps

QualifiedHigh risk

Timers

Timer relay not switching its output

A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.

5 causes3 test steps

ApprenticeLow risk