PLC output stuck on (device won't switch off)
A field device stays energised even though the program has turned the output off — the output won't release, so the device runs continuously.
Safety first
A stuck output can keep machinery running with no software control. Don't rely on the program to make it safe — isolate the device's supply. Confirm what it controls.
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
Output device failed short (relay welded / transistor shorted)
Most likelyThe physical output (relay contact or solid-state device) has failed in the conducting state.
- 2
Program still commanding it on
#2The logic is genuinely still energising the output (a latch, timer, or logic error), so it's behaving correctly to a wrong command.
- 3
Output forced on
#3The output has been left forced on in the software.
- 4
External back-feed to the device
Least likelyAnother path is feeding the device, so it stays on regardless of the PLC output.
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.
Check the program: is the output actually commanded off (and not forced)?
The output is logically off and not forced.
Logic says off but it's on — suspect a failed output or back-feed.
Program is still commanding it on (logic/latch/force) — address the logic/force.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Output stuck on
→ step 2 - 2decision
Is the output logically off and not forced?
Yes→ step 3No→ step 4 - 3decision
With OFF commanded, is the output terminal dead?
Yes→ step 5No→ step 6 - 4result
Program/force still commanding it on — fix the logic/force.
- 5decision
Is there any back-feed path to the device?
Yes→ step 7No→ step 8 - 6result
Output device failed short — repair/replace per procedure.
- 7result
Back-feed keeping it on — isolate/correct it.
- 8result
Re-examine the output device.
Common mistakes apprentices make
- Trusting the program to switch off a device whose output has welded.
- Leaving an output forced on and forgetting it.
- Not checking for an external back-feed path.
- Working on the device without isolating its actual supply.
When to stop & escalate
A failed output point means module/relay repair per site procedure. A logic error or leftover force should follow change-control. Never leave a machine running on a stuck output unattended.
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 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.
Solid-state relay (SSR) stuck on — heat won't switch off
A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.
Contactor contacts welded closed — load won't switch off
The contactor won't drop out when the coil is de-energised. The load stays powered even with the control circuit off, because the main contacts have welded together.