Misfire Monitor

Excessive engine misfire results in increased catalyst temperature and causes an increase in HC emissions. Severe misfires could cause catalyst damage. To prevent catalytic converter damage, the PCM monitors engine misfire.

The Powertrain Control Module (PCM) monitors for misfire during most engine operating conditions (positive torque) by looking at changes in the crankshaft speed. If a misfire occurs the speed of the crankshaft will vary more than normal.

The PCM can detect and compensate for variances in the engine and its components. To learn these variations, the PCM uses the input of the actual crankshaft rotation pattern and ideal crankshaft rotation pattern that has been calibrated into the PCM. The PCM then compares the two patterns. The variation between the two values is the Adaptive Numerator. If the Adaptive Numerator is not learned by the PCM, the misfire monitor will not run and the Multi-Cylinder Displacement System (MDS) will not operate. Without MDS operation, the customer will experience decreased fuel economy. If the customer experiences decrease fuel economy, use the scan tool to ensure that the Adaptive Numerator is learned.