Testing For Open/Short Circuits: General
A suitable instrument to use is a multimeter.
The ohmmeter must not be used to test components containing semi-conductors, e.g. control modules and relay with timing functions, etc.
When measuring resistance, the power supply to the system being tested must be disconnected as the measuring instrument generates a weak current in the circuit in question.
This is done to ensure that there is no current already in the tested circuit and that the correct reading is obtained.