Troubleshooting Programmable Logic Controllers (PLCs)

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.

Programmable Logic Controllers and Programming Concepts

With Electrical, Pneumatic, and Hydraulic Applications

Joji Parambath

Hardware Faults

The hardware problems in PLCs are caused by CPU, memory, I/O modules, and field devices.


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.


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 disruptions of power.

I/O Modules

The I/O modules in PLCs are provided with status indication LEDs. I/O failures are caused by errors in the PLC configuration, loose I/O blocks, broken wires, and incompatible modules.

Field devices

The field devices such as sensors and solenoids may malfunction due to loose connections, damage to the circuitry, and device failure.

Power Failure

A PLC power supply may fail due to blackout, loose connections, or broken wires. A sudden failure of the power supply can cause the scrambling of its memory and loss of process data.

Faulty Grounding

A good grounding of the PLC system is essential for the safety of the system and personnel. Ground wires must be fully intact.

Electrical Noise

A common cause of electrical noise in a PLC system is electromagnetic interference, which typically occurs, for example, when a large electric motor is started nearby.

Network and Communication Failures

The PLC communication network can fail due to hardware faults, power supply failures, wrong configuration and setting of network, and incompatible components.

Excess Heat and Vibration

A PLC system must be safeguarded from the excess heat and vibration and anything that generates excess heat should be kept at a safe distance from the PLC.

Authored by

Joji Parambath


Fluid Power Educational Series

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