JoeGuardian Automation

10. PLC Output LED ON but Device OFF (10 of 13)

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Why the PLC Says “ON” but the Field Device Does Not Energize

One of the most common troubleshooting problems in PLC-controlled panels is this:

The PLC output LED is ON,
but the field device is OFF.

This can happen with:

Solenoid valves
Motor starter coils
Interposing relays
Pilot lights
Stack lights
Alarm horns
VFD run commands
Pneumatic valves
Small contactors

A common mistake is assuming that an output LED means voltage is reaching the device.

That is not always true.

The PLC output LED usually tells you that the PLC logic is commanding the output. But it does not always prove that real voltage is leaving the output module, passing through the terminal block, traveling through the field wiring, reaching the device, and returning through the common or neutral.

The TSTrainer Lab Manual gives a similar PLC troubleshooting example where the output LEDs appear to work, but the actual field devices do not. The procedure directs the technician to measure the output points and check the power feeding the output section.

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1. The Key Concept

The most important idea is this:

PLC Output LED ON ≠ Field Device Energized

The output LED is only one part of the troubleshooting chain.

The complete chain is:

PLC Logic
   ↓
PLC Output LED
   ↓
Output Terminal Voltage
   ↓
Output Common / Fuse
   ↓
Terminal Block
   ↓
Field Wiring
   ↓
Field Device Coil
   ↓
Neutral / 0V Common Return
   ↓
Machine Action

If any part of that chain is open, missing, damaged, or incorrectly wired, the field device may stay OFF even when the PLC output LED is ON.

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2. What the Output LED Actually Means

The PLC output LED usually means:

The PLC processor is commanding the output point ON.

But it does not always confirm:

The output module has field power.
The output fuse is good.
The output common is connected.
The output transistor or relay contact is healthy.
Voltage is present at the output terminal.
The terminal block connection is good.
The field wire is intact.
The field device coil is good.
The neutral or 0V return path is complete.

That is why the multimeter is still required.

The PLC tells you what the control system is trying to do.
The meter proves what is electrically happening.

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3. Common Causes

When the PLC output LED is ON but the field device is OFF, common causes include:

Missing output common
Blown output fuse
No field power feeding the output module
Loose terminal
Broken field wire
Bad terminal block jumper
Failed output channel
Wrong output wiring
Bad interposing relay
Bad solenoid coil
Open contactor coil
Missing neutral
Missing 0VDC common
Bad connector at the field device
Device mechanically stuck
No air pressure

The important thing is not to jump directly to replacing the PLC card.

In many cases, the PLC output module is not the problem. The issue is often wiring, fuse, common, field power, or the load itself.

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4. Understand the Output Type

Before measuring, identify what type of output you are troubleshooting.

Common output types:

Output Type	Common Use	Troubleshooting Focus
Relay Output	AC/DC loads, interposing relays, coils	Check contact voltage, common feed, fuse
Sourcing Output	Sends +24 VDC to the load	Check +24V output and 0V return
Sinking Output	Switches 0V/common side	Check load feed and switched common
Triac Output	AC loads	Check leakage, load type, AC supply
Transistor Output	DC loads	Check polarity, common, short protection

This matters because not all PLC outputs work the same way.

For example:

A sourcing output supplies +24VDC to the device.
A sinking output provides the path to 0VDC.
A relay output acts like a dry contact and needs external power.

If you do not know the output type, your meter readings can be confusing.

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5. Basic 24 VDC Output Example

Example circuit:

+24VDC → PLC Output → Terminal Block → Solenoid Coil → 0VDC Common

Expected operation:

When the PLC output turns ON,
24 VDC should energize the solenoid coil.

But if the output LED is ON and the solenoid is OFF, the failure could be anywhere in this path:

Output module field power
Output fuse
Output terminal
Panel terminal block
Field cable
Solenoid connector
Solenoid coil
0VDC common return

This is why we troubleshoot step by step.

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6. Step-by-Step Troubleshooting Method

Step 1 — Confirm the output is truly commanded

First, confirm the output is ON in the PLC logic.

Check:

Output instruction
Output tag/address
Output LED
Machine mode
Permissives
Interlocks
Faults
Safety conditions

If the LED is ON, continue to electrical testing.

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Step 2 — Measure voltage at the output terminal

Measure from the PLC output terminal to the correct common or neutral.

Example for 24 VDC:

Red lead: PLC output terminal
Black lead: 0VDC common
Expected: approximately 24 VDC when output is ON

Possible results:

Reading	Meaning
24 VDC present	Output terminal is providing voltage
0 VDC present	Missing output power, bad output channel, bad common, or wrong reference
Low voltage	Voltage drop, overloaded supply, loose connection, shorted load
Voltage unstable	Loose wiring, output cycling, overload/short protection

If the output LED is ON but voltage is not present at the terminal, do not go to the field device yet. Stay in the panel and find out why.

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Step 3 — Check output common or field power

Many output modules need a separate field power feed.

The PLC processor may be powered, the output LED may turn ON, but the output device may not receive voltage if the field power/common is missing.

Check:

Output common terminal
Output fuse
Output power feed
DC breaker
Terminal jumpers
Module wiring
Power supply voltage
0VDC common

This is one of the most common causes.

Example:

PLC output LED ON
0 VDC at output terminal
24 VDC missing at output common feed

Finding:

Output module has no field power.

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Step 4 — Measure at the terminal block

If voltage is present at the PLC output terminal, move to the terminal block.

Check:

Panel side of terminal block
Field side of terminal block
Wire number
Loose screw terminal
Broken jumper
Corrosion
Mislanded wire

Expected:

Same voltage that was measured at the PLC output terminal.

If voltage is present at the PLC output but missing at the terminal block, the problem is inside the panel wiring.

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Step 5 — Measure at the field device

Now measure directly at the device.

Example:

Red lead: solenoid positive wire
Black lead: solenoid 0V common wire
Expected: 24 VDC when commanded

Possible results:

Field Device Reading	Meaning
Correct voltage present	Device coil or mechanical device may be bad
Voltage missing	Field cable, connector, terminal, or output path problem
Voltage on one wire only	Missing common/return
Low voltage	Voltage drop, bad connection, overloaded circuit

This step proves whether the field device is actually receiving usable voltage.

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7. Do Not Forget the Return Path

This is one of the biggest troubleshooting mistakes.

A load needs a complete circuit.

For DC:

+24VDC → Load → 0VDC

For AC:

L1 → Load → Neutral

A technician may measure +24VDC on one side of a solenoid and think the solenoid should energize. But if the 0VDC common is missing, the device will not work.

Always check:

Is 0VDC common present?
Is neutral connected?
Is the return wire broken?
Is the terminal loose?
Is the common fuse open?
Is the connector damaged?

A missing return path can make the output look partially correct while the device remains OFF.

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8. Example 1: PLC Output LED ON, Solenoid OFF

Symptom:

PLC output O:2/5 LED is ON.
Solenoid SV-105 does not energize.

Expected operation:

When O:2/5 is ON, SV-105 should receive 24 VDC and actuate.

Testing:

Measured 24 VDC at PLC output terminal.
Measured 24 VDC at panel terminal TB2-15.
Measured 0 VDC at solenoid connector.
Measured open conductor in field cable after LOTO.

Finding:

Broken field wire between panel and solenoid.

Correction:

Replaced damaged cable.

Root cause:

Cable was rubbing against machine frame and eventually opened.

Final verification:

SV-105 energized correctly in manual mode and automatic cycle.

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9. Example 2: Output LED ON, No Voltage at Output Terminal

Symptom:

PLC output LED is ON.
Relay CR4 does not energize.

Expected operation:

Output should provide 24 VDC to CR4 coil.

Testing:

Output LED was ON.
Measured 0 VDC at output terminal.
Measured 0 VDC at output module common feed.
Measured 24 VDC before output fuse.
Measured 0 VDC after output fuse.

Finding:

Output fuse was open.

Correction:

Replaced fuse after inspecting output wiring and relay coil.

Root cause:

CR4 coil was shorted.

Final verification:

Replaced CR4 relay.
Installed correct fuse.
Output energized normally.

This is a very important example because the PLC output LED was ON, but output field power was missing.

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10. Example 3: Voltage Present but Device Does Not Work

Symptom:

PLC output LED ON.
24 VDC present at solenoid coil.
Solenoid does not actuate.

Possible causes:

Bad solenoid coil
Stuck valve spool
No air pressure
Manual valve closed
Blocked pneumatic line
Mechanical jam
Wrong voltage coil
Weak voltage under load

Testing:

Measured 24 VDC across solenoid coil.
Checked coil resistance after LOTO.
Checked air pressure.
Manually actuated valve.

Finding:

Solenoid coil was electrically good, but valve spool was stuck.

Correction:

Replaced valve assembly.

Root cause:

Moisture and contamination in air line.

Final verification:

Cylinder actuated correctly after valve replacement and air line inspection.

This proves another key point:

Electrical output ON does not always mean mechanical action will happen.

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11. Relay Output Special Case

Relay output modules are different because the output point may be a dry contact.

A dry contact does not create voltage by itself.

It only opens or closes an external circuit.

Example:

External 120 VAC → PLC Relay Output Contact → Relay Coil → Neutral

If the PLC relay output LED is ON, the internal relay may be commanded, but you still need to verify:

Is external voltage feeding the relay contact?
Is the contact closing?
Is voltage leaving the contact?
Is the load neutral present?
Is the load coil good?

With relay outputs, a common problem is:

The PLC contact is closing,
but no external power is feeding the contact.

So the device stays OFF.

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12. Sourcing vs Sinking Output Mistake

For 24 VDC systems, wiring type matters.

Sourcing Output

A sourcing output sends positive voltage.

PLC Output → +24VDC to Load → 0VDC

Sinking Output

A sinking output switches the common side.

+24VDC → Load → PLC Output → 0VDC

If the output type and field device wiring do not match, the output LED may turn ON but the load may not energize correctly.

Check:

Module type
Wiring diagram
Load polarity
Common wiring
PNP/NPN compatibility

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13. Troubleshooting Table

Condition	Likely Problem	What to Check
Output LED ON, no voltage at output	Missing output power/common	Output fuse, field power, module common
Voltage at output, not at terminal block	Panel wiring issue	Wire, terminal, jumper
Voltage at terminal, not at device	Field wiring issue	Cable, connector, junction box
Voltage at device, device OFF	Bad load or mechanical issue	Coil, valve, relay, air pressure
Voltage on hot side only	Missing return path	Neutral or 0V common
Output turns ON then drops	Short/overload or logic issue	Load current, wiring, PLC logic
Relay output LED ON, no device voltage	No external feed	Relay contact supply voltage

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14. Technician Checklist

[ ] Confirm the output is commanded in logic.
[ ] Confirm the output LED is ON.
[ ] Identify output type: relay, sourcing, sinking, triac, transistor.
[ ] Verify output module field power.
[ ] Check output fuse or DC breaker.
[ ] Measure voltage at the output terminal.
[ ] Measure voltage at the terminal block.
[ ] Measure voltage at the field device.
[ ] Check neutral or 0VDC common return.
[ ] Inspect connector and field wiring.
[ ] If voltage is present at the device, check the load.
[ ] Check mechanical/pneumatic/hydraulic action.
[ ] Repair safely using LOTO when required.
[ ] Verify operation.
[ ] Find root cause.
[ ] Document findings.

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15. Technician Report Example

Symptom:
Reject solenoid SV-301 did not actuate during reject cycle.

Expected Operation:
PLC output O:2/5 energizes SV-301 with 24 VDC when reject command is active.

Observation:
PLC output LED O:2/5 was ON during reject command.
Solenoid did not energize.

Testing:
Measured 24 VDC at PLC output terminal O:2/5.
Measured 24 VDC at panel terminal TB4-18.
Measured 0 VDC at solenoid connector.
After LOTO, checked cable continuity and found open conductor.

Finding:
Open field wire between panel terminal TB4-18 and SV-301 connector.

Correction:
Replaced damaged field cable.

Root Cause:
Cable was pinched near conveyor frame after a recent mechanical adjustment.

Final Verification:
SV-301 energized correctly in manual mode and automatic cycle.
Reject cylinder actuated normally.

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Final Thoughts

When a PLC output LED is ON but the field device is OFF, do not assume the PLC is bad and do not assume the device should be energized.

Follow the output path:

PLC Logic → Output LED → Output Terminal → Fuse/Common → Terminal Block → Field Wiring → Device → Return Path

The PLC LED tells you the command exists.

The multimeter tells you whether voltage exists.

The field device tells you whether the machine actually responded.

A professional technician uses all three:

PLC software
Electrical diagram
Multimeter

That is how you troubleshoot PLC outputs without guessing.