Abstract: Engine diagnostic apparatus for displaying on a cathode ray tube the voltage conditions existing at each of the spark plugs which apparatus is intended for use with ignition systems in which the voltage pulses applied to the spark plugs are of such short duration that a cathode ray tube will not properly respond to the changes of voltage. The apparatus is effective to successively charge a capacitor to a value corresponding to the value of each voltage pulse and the voltages across this capacitor are used to control the vertical defection of the cathode ray tube. At the end of a first period of time following a voltage pulse, the capacitor is shorted. The capacitor is maintained shorted for a longer period of time less than the time between successive pulses so as to avoid undesired charging of the capacitor as a result of transient conditions.