Misfire Monitor
An OBD-II diagnostic program that tracks crankshaft acceleration variations to detect and log engine misfires.
The misfire monitor is a continuous diagnostic program run by the engine control unit (ECU) under OBD-II standards. Its primary function is to detect when a cylinder fails to fire (misfires), which can dump unburned fuel into the exhaust system, damaging the catalytic converter and increasing tailpipe emissions.
Rather than using individual sensors in each cylinder, the monitor utilizes the crankshaft position sensor to measure the rotational speed of the crankshaft in microsecond intervals. Every successful combustion stroke accelerates the crankshaft slightly; if a cylinder misfires, the expected acceleration pulse is missing, causing a tiny deceleration. The ECU tracks which cylinder was supposed to fire at that moment to identify and log the misfiring cylinder.
If the misfire monitor detects persistent misfires, it will illuminate the Check Engine Light and store a trouble code (e.g., P0301 for cylinder 1). If the misfires are severe enough to damage the catalytic converter, the light will flash. Mechanics use scan tools to read the misfire monitor data and freeze frame parameters to diagnose issues like bad ignition coils or clogged fuel injectors.