Misfire Detection

An engine misfire may cause damage to the catalytic converters, therefore legislation requires that control systems must be able to monitor the quality of each ignition sequence and so detect a misfire. The control system must recognize three categories of misfire:
1. A misfire which causes instantaneous catalyst damage.
2. A misfire which will cause a vehicle to fail a Federal Emissions procedure.
3. A misfire which will cause a vehicle to fail an Inspection and Maintenance test.

The misfire detection diagnostic uses the engine speed input from the crank position sensor, as its primary malfunction detection parameter.

If engine speed fluctuation is greater than the misfire detection level, a misfire counter is started. Should the counter exceed a preset criteria, a fault is flagged.