Testing For Open/Short Circuits: General
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A suitable instrument to use is a multimeter.
The ohmmeter must not be used to test components containing semiconductors, 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.