Digital multimeter (DMM)
Definition
The digital multimeter measures voltage, resistance, current and often frequency, duty cycle, diode drop and temperature, and is the core tool for electrical diagnosis; an automotive-grade meter has an input impedance of at least 10 megohm so it does not load sensor circuits, a min/max or peak-capture function to catch intermittent dropouts, and a clamp accessory for starter, alternator and parasitic current without opening the circuit. Voltage tests are made with the circuit live and loaded (supply at the component, voltage drop across cables and grounds, sensor signal while the engine runs); resistance tests only on de-energised circuits and only for components with a specified value (coils, injectors, heaters, temperature sensors), remembering that a circuit can show correct resistance and still fail under current. Typical reference values: battery 12.6 V at rest, 13.8-14.8 V charging, less than 0.2 V drop on a main cable, lambda heater 2-15 ohm, port injector 12-16 ohm, ignition coil primary 0.5-1.5 ohm, CAN termination 60 ohm, NTC temperature sensor 2-3 kohm at 20 °C. A multimeter averages fast signals, so for crank and cam sensors, injector waveforms, CAN and fast-switching solenoids an oscilloscope is needed instead. Related codes: supports diagnosis of circuit codes (P0xx1-P0xx4 open/short/high/low), heater codes P0030-P0038, P0135, P0562/P0563 charging, U0xxx bus faults and any 'circuit' DTC.