PLC outputs are the machine's hands — commands that energise contactors, open solenoids, start drives and light indicators. When an output fault occurs, the machine either fails to actuate or does the wrong thing. This guide covers a structured approach.

A structure for output faults

  1. Is the PLC commanding the output? Check the logic status and the rung driving it.
  2. Is the output status physically present? Does the output LED light, and is there voltage at the terminal?
  3. Is the field device being driven? Is the contactor pulling in, the solenoid shifting, the drive receiving its run command?
  4. Are permissives or interlocks holding the output off? A rung may be conditioned by a safety or sequence interlock.
  5. Is the field device healthy? Coil, solenoid, drive command wiring.

Common output types

  • Relay outputs — driving contactors, solenoids, indicators. Watch for welded or worn relay contacts.
  • Transistor outputs — fast switching, often driving VFD/servo command signals or indicators.
  • Analog outputs — speed or position references to drives; check for scaling and signal health.

An illustrative diagnostic flow

  • START — device does not actuate
  • ↓ Is the logic energising the output? → No → check the rung, conditions and mode
  • ↓ Yes → Is the output LED on? → No → possible output card fault or configuration
  • ↓ Yes → Is voltage present at the terminal? → No → fuse/blown output
  • ↓ Yes → Is the field device receiving it? → No → wiring/connector
  • ↓ Yes → Is the field device itself healthy? → No → replace coil/solenoid/drive-side
  • ↓ Capture and document the cause

Key points

  • Separate "the PLC is not commanding" from "the PLC is commanding but nothing happens" — they point to completely different areas.
  • Test the output, the field device and the wiring; do not assume the card.
  • Some PLC brands are compatible with specific field devices and some are not — do not assume interoperability without checking the manufacturer documentation.