Abstract: A system and method for diagnosing ignition system performance of an internal combustion engine. The method operates an engine control unit to produce a sequence of spark ignition current pulses through primary windings of a plurality of ignition coils. The current pulses produce voltage pulses across electrodes of the spark plugs. The voltage pulses produce flyback voltages across the primary windings. Output pulses are produced in response to the flyback voltages having time durations related to the time durations of the sparks produced by the spark plugs. Digital signal are produced in response to each one of the output pulses representative of the spark time duration of a corresponding one of the sparks. A comparison of the digital signals determines whether the spark time duration of one of the spark plugs is substantially different from the spark time durations of the other ones of the plurality of spark plugs.