Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The PCM determines crankshaft rotational velocity using the crankshaft position sensor and camshaft position sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the PCM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to monitor a misfire present for between 1000-3200 engine revolutions.

For catalyst-damaging misfire, the diagnostic will respond to misfire within 200 engine revolutions.

Rough roads may cause false misfire detection. A rough road will cause torque to be applied to the drive wheels and drive train. This torque intermittently decreases the crankshaft rotational velocity. This may be falsely detected as a misfire.

On automatic transmission-equipped vehicles, a rough road sensor, or G-sensor, works together with the misfire detection system. The G-sensor produces a voltage that varies along with the intensity of road vibrations. When the PCM detects a rough road, the misfire detection system is temporarily disabled.