The PCM uses pulses from the crankshaft position sensor to determine the position and RPM of the engine at various degrees of rotation. The PCM can then base misfire diagnostics on the principle that crankshaft velocity will increase as each cylinder contributes it power input. Under certain conditions the PCM will perform a series of tests to determine misfire percentage on a cylinder based on crankshaft velocity variation. The MIL, service engine soon, light will flash if an active catalyst damaging misfire occurs (20-100 percent) and will illuminate ON solid if an emission related misfire (2-19 percent) is present after 2 consecutive trips. The MIL, service engine soon, light will turn ON after 3 consecutive passed trips meeting the same misfire criteria or 80 passed trips if the criteria has not been met.

Important: The misfire diagnostics are continually checked with the engine running when the above conditions have been met.

DTC P0301, P0302, P0303, or P0304 will set if a misfire has been detected after a series of tests when:

If an injector, IC, or EGR DTC is active and the misfire DTC is active, diagnose the other DTCs first.

Use the scan tool to monitor active misfire counts. Spraying water on the wires provides a good path for current to flow to ground on a bad wire. The scan tool active misfire counts should pick up an intermittent misfire problem. Watch the misfire counter. If a specific cylinder misfires under certain load conditions, it may be duplicated in the stall.