Misfire Monitor Diagnostic Tests

When the engine is running and a cylinder misfires the crankshaft slows down momentarily. The control module has the ability to detect this change in speed by monitoring the input signals from the following sensors:
^ The crankshaft position (CKP) sensor
^ The camshaft position (CMP) sensor.

When the control module is running the misfire on-board diagnostic tests and a misfire is detected the misfire counter stores (accumulates) the position and the number of misfires. Two counters (accumulators) are assigned to each of the cylinders. The counters are as follows:
^ The Current Misfire Counters. The current misfire counters store the number misfires that have occurred in the last 200 engine cycles. When the control module detects an unacceptable amount of misfires stored in a current misfire counter the misfire on-board diagnostic test fails and a misfire DTC will set.
^ The History Misfire Counters. When a misfire DTC is set, the history misfire counters store the total number of misfires that have occurred every 200 engine cycles.