WiremanWireman · PLC output not working: troubleshooting for electricians · 2023 NEC ·
PLC output not working
LED versus actual voltage at the terminal, then the field wiring and the load.
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How it works
A PLC output is a switch the program controls: a relay contact, a transistor (DC) or a triac (AC). The output doesn’t make power on its own; the card’s common or supply terminal has to be fed, and the output passes that to the load. The output LED shows what the program commanded, not that power reached the load. So compare the LED, the voltage at the output terminal, and the load.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When the output should be on, is its LED lit?
- LED on, load doesn’t runTest: Measure at the output terminalExpect: The load voltage (often 120 V AC or 24 V DC) at the output terminal when on.
- No voltage on the card’s supply or commonOutput group has no supply
- Supply present, output terminal deadFailed output point, fuse or card
- Voltage at the terminal, load still offTest: Measure at the loadExpect: The same voltage you read at the output terminal.
- No voltage at the loadOpen wiring between the card and the load
- Full voltage, load doesn’t workThe load has failed
- LED offOnline with the PLC, is the output’s coil in the program true?
- Rung is falseA condition in the program isn’t met
- Rung true, LED still offFailed output point, fuse or card
- PLC in Program mode or faultedPLC isn’t running
- Load stays on with LED offOutput leakage or a failed-on output
- LED on, load doesn’t runTest: Measure at the output terminalExpect: The load voltage (often 120 V AC or 24 V DC) at the output terminal when on.