For any PLC controlled plant, by far the greater percentage of the faults are likelly to be with sensors, actuators, and wiring rather than with PLC itself. The faults within the PLC most are likely to be in the input/output channells or power supply than in the CPU.
Consider a single output device failing to turn on though the output LED is on.
If testing of the PLC output voltage indicates that it is normal then the fault might be a wiring fault or a device fault.
If checking of the voltage at the device indicates the voltage there is normal then the fault is the device.
Failure of an input LED to illuminate as required could be because:
*Input device is not correctly operating,
*Input device is not correctly powered,
*Incorrect wiring connections to the input modulle, or LED or input module is defective.
Many PLCs provide built-in fault analysis procedures which carry out self-testing and display fault codes, with possibly a brief message, which can be translated by looking up code in a list to give the source of the fault and possible method of recovery.