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 its 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 OFF 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 following conditions have been met.

DTC P0300 will set if random misfires have been detected after a series of tests when:

Verify the crank learned procedure has matched the PCM to the crankshaft in the vehicle.

If an injector, EST, or EGR DTC is active and a misfire is active, diagnose those 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 scan tool misfire counter. When a specific cylinder misfires under certain load, conditions may be duplicated in the stall.