Troubleshooting Programmable Logic Controllers by Joji Parambath A PLC system is usually designed with LEDs on the CPU, power supply, and I/O modules for fault diagnosis. It is usually provided with built-in fault analysis measures to carry out self-testing and display fault codes, which can then be interpreted by looking up the code in a list provided by the manufacturer to give the source of the fault and possible method of rectification. Moreover, the PLC may be provided with a backup power source to keep the system running in the event of a power failure. A PLC system may fail due to the following reasons: hardware problems, alteration of user programs, improper wiring, power failures, ground integrity, electromagnetic interference, loss of communication, and excess heat. Hardware Faults The hardware problems in PLCs are caused by CPU, memory, I/O modules, and field devices. CPU The troubleshooting engineer must identify the source of the problem using LED indicators, and the troubleshooting table indicating the problem description, probable causes and recommended action against each status indication. Memory The PLC memory can become corrupted, or the program may get altered by external factors such as electromagnetic interference, radio frequency interference, improper wiring and grounding, heat, and power failure.