A dead channel on a PLC input module is not always a dead module. Wiring faults, field power loss and sensor failures produce identical symptoms. Run these five checks in order — most "module failures" turn out to be field wiring.
| Check | What to look for | Verdict |
|---|---|---|
| 1. Module LEDs | Channel LED lit when sensor active? | No LED = field-side fault |
| 2. Terminal wiring | Loose screw, wrong terminal, swapped pair | Most common cause |
| 3. Field power | 24 V DC present at the terminal? | Measure before blaming the module |
| 4. Sensor output | Sensor switching output actually toggles? | Test sensor at the terminal block |
| 5. CPU / rack status | Bus fault, module missing, force flags | Forces can fake a "dead" channel |
Every digital input module has per-channel LEDs. If the LED is dark while the sensor is definitely active, the problem is upstream (wiring, power, sensor). If the LED lights but the program sees no input, the fault is downstream (module logic or CPU mapping).
Vibration loosens screw terminals. Retorque every terminal on the affected channel group and check for swapped signal pairs — a single mis-landed wire produces a permanent "no signal" that mimics module failure.
Put a meter on the terminal block: is 24 V DC actually present at the sensor commons? Blown fuses and tripped distribution breakers are a top-3 cause of "failed" input modules.
Bypass the sensor: jumper the input terminal to the common. If the module then registers the input, the sensor (or its cable) is the culprit — not the module.
Leftover forces from commissioning make a channel look dead in the program. Clear all forces, check the module's status in the hardware configuration, and look for bus-fault codes in the diagnostic buffer before ordering hardware.
If all five checks pass and the channel stays dead — or multiple adjacent channels fail together — the input circuit is damaged. We stock tested PLC input modules for Siemens S7-300/1200/1500, Allen-Bradley SLC 500 and CompactLogix, Mitsubishi FX and Omron CJ/NX.
Almost certainly not. Total channel loss points to field power or common wiring — check the 24 V supply first.
Component-level repair is rarely economical; a tested replacement module is faster and carries a 12-month warranty.
Yes — every module is bench-tested on all channels with a signal simulator before it ships.