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An onboard diagnosis method for misfire detection that utilizes a superposed output of combustion pressure sensors (CPS's) is proposed and examined. We consider a model case where piezo-electric type CPS's are installed for each cylinder of a four-stroke-four-cylinder gasoline engine. The outputs of the CPS's are linearly combined to obtain the superposed signal. We show that the superposed signal exhibits a characteristic change when a misfire occurs. A waveform analysis of the signal leads to a clear detection of misfires. A combination setup with a non-intrusive type of fiber-optic CPS that is installed into the cylinder head gasket is also discussed as a future option for the sensing system.