When testing resistance in a circuit with an ohmmeter, what must be done first?

When measuring resistance in a circuit using an ohmmeter, the first step is to ensure that the component being tested is disconnected from the power source. This is crucial because an ohmmeter operates by sending a small current through the component to measure its resistance. If the circuit is powered on or connected, it can create misleading readings and potentially damage the ohmmeter due to the voltage present in the circuit.

Disconnecting the component from the power source ensures that the ohmmeter can accurately assess the resistance without interference from any live circuits. This step not only protects the instrument but also provides a clear measurement of the component’s resistance, allowing for effective diagnostics in automotive electrical systems.