What happens to the readings if an ohmmeter is used while the circuit is powered?

Using an ohmmeter while a circuit is powered can lead to erratic readings because an ohmmeter applies a small voltage to measure resistance, and this process requires the circuit to be de-energized. When the circuit is energized, the presence of voltage can interfere with the ohmmeter's ability to accurately measure resistance. The energized components can create a situation where the ohmmeter receives mixed signals—both from the circuit's powered state and from its own testing voltage—resulting in fluctuating or inconsistent resistance values.

This is why it is critical to ensure that all power is removed from a circuit prior to measuring resistance with an ohmmeter to obtain reliable and stable readings. In addition, using an ohmmeter in a powered circuit can also potentially damage the meter, complicating the diagnostics further. This is why understanding the correct procedure for using testing equipment is essential in automotive electrical diagnostics.