The first job is not to make the problem worse, by jumping in with two left feet.
Understanding how a PLC works and using a logical approach is the key to sucessfully fault finding a PLC systems.
Some fairly simple techniques can be used to fault find the PLC. The easiest is probably the half split technique. This however relies on knowing where the various check points are for the PLC system.
Our courses are based on understanding simple diagrams and then using them to fault find, delegates cannot believe how simple it is once you know where the check points