Abstract: A misfire monitor for detecting cylinder misfires in an internal combustion engine. The monitor has a light detector that senses infrared light signals emerging from combustion gases through a window inserted on an exhaust pipe or manifold. Characteristics of a detected light signal indicate whether there is a firing or a misfire in any cylinder. Other information about the engine is deduced with processing of the detected signals and misfire counts. The detection is real time and functions well at all normal engine speeds and operating conditions.