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An in-car warning system has been designed which indicates the occurrence of excessive engine misfires by analyzing the ionization signals obtained from each spark plug's center electrode. A prototype model of this design has been built and evaluated on a 2000 cc, 1973 production four-cylinder engine. Reliable operation was achieved from idle to 6000 rpm. The system consists of two basic elements, adapters for each cylinder that fit into the distributor cap for obtaining the combustion signals, and a simple electronic circuit for analyzing these signals and activating a warning light. Since the adaptors consist only of a 20 MΩ isolation resistor and a connector, they could easily be designed as an integral part of the distributor cap. Since the instrument will detect excessive misfiring conditions, including those caused by broken or shorted spark plug wires, and has the self-checking feature of turning on the indicator lamp for a short period of time when the ignition switch is engaged, it should be useful as a warning system to inform the operator that the engine is in need of repair.