PLC input stays on after the sensor has switched off
A digital input LED (and the PLC bit) stays on even after the sensor/switch has clearly released. The logic thinks the input is still made, so the sequence stalls or behaves wrongly.
Safety first
A stuck input can make a machine behave unexpectedly. Treat the machine as able to move, keep clear of moving parts, and isolate/secure it before working on field wiring.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Leakage current holding the input on
Most likelyThree-wire sensors and long cable runs leak a small current. With a high-impedance input it can stay above the off-threshold, so the input never drops out. A bleed resistor or different input type may be needed.
- 2
Welded/stuck field contact
#2A relay or switch contact feeding the input that's welded closed keeps the input made regardless of the actual condition.
- 3
Wiring short to the supply rail
#3A conductor chafed to the +24V (or 0V on sinking) keeps the input energised independent of the sensor.
- 4
Forced bit or logic latch in the program
Least likelyAn input forced in the software, or a latch/seal in the logic, can make a healthy input appear stuck 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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeMeasure the voltage at the input terminal with the sensor released and compare to the input's off-threshold.
Input voltage well below the off-threshold when the sensor is off.
Field signal is genuinely off but the bit is on — suspect a forced bit or logic latch.
Voltage still present at the terminal with the sensor off — it's leakage or a wiring/contact fault keeping it energised.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
PLC input stuck on after sensor off
→ step 2 - 2decision
Is voltage at the input below the off-threshold with the sensor off?
Yes→ step 3No→ step 4 - 3decision
Does the input drop out when the field wire is removed?
Yes→ step 5No→ step 6 - 4result
Field signal still present — leakage / short / stuck contact in the field wiring.
- 5result
Fault is in field wiring/sensor — but check program forces too.
- 6result
Module fault or forced/latched bit — clear forces/logic or replace the module.
Common mistakes apprentices make
- Blaming the PLC module when leakage from a three-wire sensor is holding the input on.
- Forgetting to check for forced bits in the program.
- Not disconnecting the field wire to split field vs module faults.
- Adding a 'fix' in software to mask a stuck field contact.
When to stop & escalate
If a welded field contact is feeding a safety-related function, treat it as a safety issue and escalate. Leakage problems on long sensor runs may need an input-type/design change — flag rather than bodge.
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.
Was this guide helpful?
Related faults
PLC input not reading despite a signal present
A field device is clearly providing a signal, but the PLC input bit/LED doesn't come on — the program never sees the input, so logic that depends on it won't run.
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.
Control relay contacts sticking or welded
A control/interface relay's contacts stick closed (or won't make cleanly) — the controlled circuit stays on when it should be off, or switches unreliably.