AdvancedHigh risk

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.

Start fault-finding

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.

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

Likely causes

Ranked from most to least likely.

1
Mode switch in STOP/PROG
Most likely
The controller's key/mode switch is in stop or program, not run.
2
Major fault halted the CPU
#2
A program or hardware major fault has put the CPU into a faulted/stopped state.
3
Watchdog / scan-time fault
#3
A watchdog timeout or excessive scan time has faulted the controller.
4
I/O or module fault escalated to a CPU fault
#4
A configured I/O fault has been set to stop the CPU.
5
Power / battery / memory issue
Least likely
A power event, failed memory backup, or corrupted program left it stopped.

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

Premium fault tree

The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Testing sequence

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

Test 1 of 3

Check the controller's mode switch and read the exact fault/diagnostic from the controller.

Expected reading

Mode in run (or a clear fault explaining the stop).

Your actual reading

If it passes

Mode is run with a logged fault — work the specific fault.

If it fails

Mode in stop/prog — confirm it's safe, then return to run.

Often leads toPLC I/O module faulted / showing module error PLC communication fault (network / remote I/O)

View all expected readings at once

1. Check the controller's mode switch and read the exact fault/diagnostic from the controller.

Mode in run (or a clear fault explaining the stop).

2. Review the fault diagnostics: major fault, watchdog/scan, or I/O-triggered stop.

A specific fault cause to act on.

3. Address the identified cause (clear the fault per procedure), ensure the area is safe, and return to run.

Fault clears and the program runs with I/O controlled.

Full test sequence

The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Fault-finding flowchart

The same logic as a decision tree.

1
start
PLC in fault/stop
→ step 2
2
decision
Is the mode switch in run with a logged fault (vs simply in stop)?
Yes→ step 3No→ step 4
3
decision
Is a specific fault cause identified?
Yes→ step 5No→ step 6
4
result
Mode in stop/prog — confirm safe, then return to run.
5
result
Address the cause per procedure, ensure safe, then run; escalate if it re-faults.
6
result
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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General testing

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.

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Learn the theory

How the gear and circuits behind this fault actually work.

Device

PLC (Programmable Logic Controller)

An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.