PLC input not reading despite a signal present
A field device is clearly providing a signal, but the PLC input bit/LED doesn't come on — the program never sees the input, so logic that depends on it won't run.
Safety first
Forcing inputs to test can make the program act as if a device operated. Understand what depends on the input before forcing anything.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Signal not reaching the input terminal
Most likelyA broken wire, loose terminal, or open device contact means the signal never arrives at the input.
- 2
Sourcing/sinking (PNP/NPN) or common mismatch
#2The field wiring or device type doesn't match the input module's sourcing/sinking arrangement or its common.
- 3
Missing input common / reference
#3The input group's common/reference isn't connected, so no input in that group can register.
- 4
Failed input channel
#4The specific input channel has failed and won't read even a good signal.
- 5
Wrong addressing / mapping in the program
Least likelyThe signal is read on a different address than the logic expects, so it 'doesn't read' where you're looking.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure the input signal at the PLC input terminal (relative to the input common) when the device operates.
The expected on-state voltage at the input terminal.
Signal reaches the terminal but the bit stays off — check common, channel, addressing.
No signal at the terminal — trace the field wiring/device back.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Input not reading
→ step 2 - 2decision
Is the expected signal present at the input terminal?
Yes→ step 3No→ step 4 - 3decision
Is the input common connected and PNP/NPN type matched?
Yes→ step 5No→ step 6 - 4result
No signal at the terminal — trace the field wiring/device.
- 5decision
Does a known signal turn on the bit at the expected address?
Yes→ step 7No→ step 8 - 6result
Missing common or type mismatch — correct the wiring.
- 7result
Re-check the field signal path.
- 8result
Channel fault or wrong addressing — repair/replace or remap.
Common mistakes apprentices make
- Measuring without referencing the correct input common.
- PNP/NPN mismatch between device and input module.
- Forgetting the shared input common for the whole group.
- Looking at the wrong address in the program.
When to stop & escalate
A failed input channel means controller maintenance per site procedure with program backups. Persistent wiring-type mismatches suggest reviewing the I/O design.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
Limit switch or proximity sensor not being detected
A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.
PLC I/O module faulted / showing module error
An I/O module shows a fault LED or the controller reports a module error — a whole block of inputs/outputs is dead or unreliable, not just one channel.
Learn the theory
How the gear and circuits behind this fault actually work.