PLC Output LED Is On, but Nothing Moves: Follow the Missing Power Path
A true output command is only one link in a machine's power path. The load may still lack its supply, return, interface relay, or permitted operating condition. Separate the software request from the electrical output and the physical response instead of treating an output LED as proof of delivered power.
General troubleshooting; terminal assignments and diagnostic meanings are module-specific.
What is happening?
Output modules differ: relay contacts, sourcing transistors, and sinking transistors do not complete the circuit in the same way. Some indicators show command status rather than verified load current. The module's logic supply may also be separate from the field supply feeding a group of outputs.
Check these five things
-
Identify the output type and the LED's documented meaning. Confirm the exact channel and load from the approved circuit drawing.
-
Check controller mode, module diagnostics, output inhibits, and configured fault behavior. A displayed command may not be the value currently applied to hardware.
-
Trace the field-supply and return path on the drawing, including group fuses, terminal links, interface devices, and safety-controlled power removal.
-
Have authorized personnel compare the relevant electrical conditions using the approved procedure and properly rated equipment. Record the commanded state at the same time.
-
If the load receives the specified conditions but does not respond, move the investigation to the load and mechanism. Do not keep changing PLC logic to compensate.
Worked example
A training panel's output tag and indicator are on, but the associated relay coil has no field supply because its output group is separately fused. The useful finding is the missing supply path. The fault is not established merely by measuring an unrelated 24 V point elsewhere in the cabinet.
Quick diagnostic reference
| Layer | Evidence to collect |
|---|---|
| Logic request | Final command and active mode |
| Output hardware | Diagnostics and field-supply status |
| Load | Specified electrical conditions and physical response |
The mistake to avoid
Never bridge an interlock, fuse, or safety-controlled supply to prove that an actuator can move. Use the machine's authorized test method.
How to verify the fix
Confirm the original command, electrical output, and load response agree, then check the normal off state and document the actual repaired fault.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Rockwell Automation technical documentation: select the exact I/O module manual. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.