Why This Problem Is So Common 

Seeing an output energized in the PLC while nothing happens in the field is one of the most common troubleshooting scenarios in industrial automation. 

What the Output LED Actually Means 

The LED only indicates the PLC logic has turned the output on. It does not confirm field voltage or device operation. 

Most Common Causes 

• Missing field power 

• Incorrect sourcing/sinking wiring 

• Missing or incorrect commons 

• Welded or failed relays 

• Blown interposing fuse 

How to Troubleshoot It 

• Verify voltage at the output terminal 

• Check the load side of the device 

• Trace the circuit using the print 

• Confirm commons and references 

Final Thought 

The PLC is often telling the truth. The disconnect is usually between the output terminal and the device.