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A method of monitoring the quality of combustion in spark ignition gasoline engines using the spark plug's center electrode as a plasma probe is presented. Due to the ionized species remaining in the burned gases after combustion, a current is induced in a network attached to the spark plug's center electrode. The time dependence of this induced current is shown to correlate with the presence or absence and the time of occurance of the pressure component due to combustion in pressure transducer signals recorded simultaneously. Based on these correlations, three types of burns could be recognized from plasma probe signals: good burns, slow burns, and misfires. The specific correlation between the absence of a pressure component due to combustion and a corresponding absence of a plasma probe signal was used to form the basis for the operation of a one-channel engine misfire monitoring circuit.