Abstract: The secondary ignition circuit of a spark ignited internal combustion engine is analyzed to identify the location of faulty portions of the circuit, the circuit having an induction coil for generating sparking voltage pulses repetitively, and a distributor for cyclically distributing the voltage pulses to respective spark plugs in a predetermined order. The peak amplitude of respective voltage pulses of one polarity developed at a single point in the secondary ignition circuit for respective spark plugs in each engine cycle is measured to produce measurement signals systematically related thereto. A comparison signal is derived that is systematically related to the average peak amplitude of voltage pulses of the one polarity developed for all spark plugs. Successive measurement signals are compared with the comparison signal, and a fault is indicated when a measurement signal differs from the comparison signal by more than a predetermined differential.