Misfire Monitor Diagnostic Operation

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

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1,000 to 3,200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

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