PLC inputs / outputs
Inputs not reading, outputs on but device dead.
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.
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.
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.
PLC communication fault (network / remote I/O)
The PLC has lost communication with a device, remote I/O, HMI, or network — comms-fault indication, missing data, or remote I/O dropping out.
PLC output stuck on (device won't switch off)
A field device stays energised even though the program has turned the output off — the output won't release, so the device runs continuously.
PLC in fault / stop mode (not running the program)
The PLC has stopped running its program — a fault LED is on or it's in STOP/PROG mode — so no I/O is being controlled and the machine is dead in a defined way.
PLC analogue output not driving correctly
A PLC analogue output (to a valve positioner, VSD reference, etc.) isn't producing the right signal — the device doesn't follow the command, or the output sits at zero/full scale.
HMI / touchscreen blank or frozen
The operator HMI/touchscreen is blank, frozen, or unresponsive — operators can't see status or control the machine, even though the PLC may still be running.
PLC output fuse keeps blowing
The fuse protecting a PLC output (or output group) blows repeatedly — knocking out one or several outputs each time it goes.
PLC output flickering / device chattering
A PLC output rapidly switches on and off, making the connected device (contactor, valve, lamp) chatter or flicker instead of staying in a steady state.