Abstract: The invention includes circuitry adapted to be interconnected with the ignition system of spark-fired internal combustion engines. The circuit created is adapted to shut off the engine by preventing induction coil voltage buildup by grounding the secondary circuit of the coil when the engine overheating switch closes or, in the alternative, when an antitheft switch similarly positioned in the circuit is closed. The engine shutoff provided is in addition to the signal light which when energized indicates that the engine is overheating.A method of preventing voltage buildup in the induction coil by grounding the secondary circuit of the coil in an ignition system when the heat sensing switch, or a normally open antitheft switch, is closed.