QualifiedMedium risk

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.

Start fault-finding

Safety first

Field devices on PLC outputs can move machinery the moment the circuit completes. Confirm what the output controls and secure the area before forcing or bridging anything.

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. 1

    Open circuit between output and device

    Most likely

    A broken wire, blown output fuse, loose terminal, or open in the field wiring means the output is 'on' at the card but nothing reaches the device.

  2. 2

    Missing field/load supply

    #2

    Many output cards switch a separate field supply. If that supply (or its common/return) is missing, the LED can still show logic state while the load gets nothing.

  3. 3

    Failed output channel (won't actually switch)

    #3

    The output's switching element has failed open. The logic LED tracks the program, not the real output, so it lies about the actual state.

  4. 4

    Faulty field device or its own protection

    #4

    The valve coil, contactor coil, or lamp has failed, or a downstream fuse/overload for the device has opened.

  5. 5

    Wrong output type / load mismatch

    Least likely

    A device drawing more than the output can drive, or a mismatch between sourcing/sinking wiring, can leave the device unpowered.

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.

Test 1 of 4
1

With the output commanded on, measure voltage at the output terminal to its common/return.

Expected reading

The expected switching voltage present at the output terminal when on.

If it passes

The card is switching — the fault is downstream in the field wiring or device.

If it fails

No voltage at the output despite the LED means a failed output channel or a missing field supply.

Often leads toTimer relay not switching its outputNo control voltage in the panel
View all expected readings at once
1. With the output commanded on, measure voltage at the output terminal to its common/return.
The expected switching voltage present at the output terminal when on.
2. If no output voltage, confirm the field/load supply and its common are present and the output's fuse is intact.
Field supply and common present; output fuse healthy.
3. If output voltage is present, measure voltage right at the device terminals.
The same switching voltage arriving at the device.
4. Isolate, then test the field device (e.g. coil resistance) and its downstream protection.
A healthy coil/load and intact downstream fuse/overload.

Fault-finding flowchart

The same logic as a decision tree.

  1. 1
    start

    Output LED on, device dead

    → step 2
  2. 2
    decision

    Is switching voltage present at the output terminal?

    Yes→ step 3No→ step 4
  3. 3
    decision

    Does that voltage also reach the device terminals?

    Yes→ step 5No→ step 6
  4. 4
    decision

    Is the field supply/common present and the output fuse intact?

    Yes→ step 7No→ step 8
  5. 5
    result

    Field device or its downstream protection has failed. Test the coil/load.

  6. 6
    result

    Open in the field wiring between card and device. Find the break.

  7. 7
    result

    Output channel has failed to switch — plan a card repair/replace.

  8. 8
    result

    Missing field supply/common or blown fuse. Restore it.

Common mistakes apprentices make

  • Trusting the output LED as proof the output is really switching — it follows the logic, not the physical output.
  • Forgetting the separate field supply/common that the output card switches.
  • Not measuring voltage right at the field device, missing a break between the card and the device.
  • Forcing the output in software without confirming what it physically moves.

When to stop & escalate

If the output channel itself has failed, plan a card swap/repair per site procedure and program backup. If forcing outputs is involved, follow the site's change-control and never leave an output forced on a running machine 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

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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.