A digital multimeter is the first diagnostic tool worth owning for automotive and electrical work, measuring voltage, current, resistance and continuity, with clamp versions adding current measurement without breaking the circuit.

For vehicle work the meter earns its keep on voltage drop testing, which is how you find the corroded connections and poor earths behind most intermittent electrical faults. Measuring the drop across a live circuit tells you far more than a resistance check on a disconnected one.

Buy on safety rating rather than features. The CAT rating and voltage figure define what the meter can survive, and a cheap unrated meter is genuinely dangerous on anything beyond low-voltage DC. Check the leads for damage before use, start on the highest range if you are unsure, and never measure resistance on a live circuit.