Abstract: A method of testing spark plugs in an engine is described. The engine is driven by its starter motor or other motor while ignition voltage is applied, the throttle is open and the fuel is cut off. This loads the spark plugs to reveal defects not otherwise apparent. Abnormally high or low plug peak voltages as well as abnormal arc durations are detected and counted and the counts are displayed. A microprocessor based measuring instrument coupled to capacitive pickups adjacent the spark plug wires and to the engine distributor evaluates the spark voltage and duration characteristics and displays the fault data.