Proximity sensor permanently on (always made)
A proximity sensor reads 'detected' all the time — its output stays on even with no target present, so the controller always sees the input made.
Safety first
A stuck-on sensor can make a machine think it's in a position it isn't. Confirm actual position before moving anything based on the input.
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
Target/metal too close or fixed metal in range
Most likelyFixed metal within the sensing range, or the gap set too small, keeps an inductive prox permanently triggered.
- 2
Sensor output short / failed on
#2The sensor's output transistor has failed in the conducting state, holding the signal on.
- 3
Wiring short to the active line
#3A cable short or pinched wire holds the input line at the active level regardless of the sensor.
- 4
Wrong sensor type for the application
Least likelyA normally-closed (NC) sensor used where NO logic is expected reads as always-on.
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.
Remove all targets and check the sensor's indicator LED and output state.
Output turns off with no target in range.
Sensor releases — your 'always on' was a present target; check positioning.
Stays on with nothing present — check for fixed metal, then the sensor/wiring.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Prox always on
→ step 2 - 2decision
Does the output turn off with no target present?
Yes→ step 3No→ step 4 - 3result
It was a present target — check positioning/gap.
- 4decision
Is there fixed metal in range or the gap too small?
Yes→ step 5No→ step 6 - 5result
Reposition / adjust the gap.
- 6decision
Does the input clear when the sensor is disconnected?
Yes→ step 7No→ step 8 - 7result
Sensor failed on — replace (confirm NO/NC).
- 8result
Wiring short to the active line — find and repair.
Common mistakes apprentices make
- Not realising fixed nearby metal keeps an inductive prox triggered.
- Confusing an NC sensor's 'always on at rest' behaviour with a fault.
- Blaming the sensor when a pinched cable shorts the line.
- Setting the sensing gap too small.
When to stop & escalate
If a stuck input affects a safety or positioning interlock, treat position confirmation as critical before any movement. Replace a failed sensor rather than working around it.
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
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.
Sensor intermittent / drops out randomly
A sensor works most of the time but drops out or false-triggers intermittently, causing random stops, miscounts, or sequence faults that are hard to pin down.
Limit switch stuck made (won't release)
A mechanical limit switch stays operated even after the actuator leaves it — the input stays made, so the machine thinks it's still at that limit.
Learn the theory
How the gear and circuits behind this fault actually work.