PLC in fault / stop mode (not running the program)
The PLC has stopped running its program — a fault LED is on or it's in STOP/PROG mode — so no I/O is being controlled and the machine is dead in a defined way.
Safety first
When the PLC returns to run, outputs can energise and machinery move. Ensure the area is clear before clearing a fault or switching to run. Follow site procedures and back up before changes.
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
Mode switch in STOP/PROG
Most likelyThe controller's key/mode switch is in stop or program, not run.
- 2
Major fault halted the CPU
#2A program or hardware major fault has put the CPU into a faulted/stopped state.
- 3
Watchdog / scan-time fault
#3A watchdog timeout or excessive scan time has faulted the controller.
- 4
I/O or module fault escalated to a CPU fault
#4A configured I/O fault has been set to stop the CPU.
- 5
Power / battery / memory issue
Least likelyA power event, failed memory backup, or corrupted program left it stopped.
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 ModeCheck the controller's mode switch and read the exact fault/diagnostic from the controller.
Mode in run (or a clear fault explaining the stop).
Mode is run with a logged fault — work the specific fault.
Mode in stop/prog — confirm it's safe, then return to run.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
PLC in fault/stop
→ step 2 - 2decision
Is the mode switch in run with a logged fault (vs simply in stop)?
Yes→ step 3No→ step 4 - 3decision
Is a specific fault cause identified?
Yes→ step 5No→ step 6 - 4result
Mode in stop/prog — confirm safe, then return to run.
- 5result
Address the cause per procedure, ensure safe, then run; escalate if it re-faults.
- 6result
Check power/battery/memory status for the cause.
Common mistakes apprentices make
- Switching to run without checking the area is safe (outputs energise).
- Clearing a major fault without understanding why it occurred.
- Not backing up before making any program/config change.
- Missing a simple mode switch left in stop/program.
When to stop & escalate
Major/program faults that recur should go to the controls/automation team with the diagnostics captured. Always back up the program before changes and follow site change-control.
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.
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Related faults
PLC I/O module faulted / showing module error
An I/O module shows a fault LED or the controller reports a module error — a whole block of inputs/outputs is dead or unreliable, not just one channel.
PLC communication fault (network / remote I/O)
The PLC has lost communication with a device, remote I/O, HMI, or network — comms-fault indication, missing data, or remote I/O dropping out.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.