P0303

Circuit Description
The Crankshaft Position (CKP) Sensor sends pulses to the ECM as the reluctor wheel teeth rotate past the sensor. Besides using these pulses in order to synchronize the ignition and fuel injector operation, the ECM also times the interval between each pulse and compares each new time interval with the previous one in order to determine when an excessive change in the crankshaft speed has occurred. A misfire causes an unexpected change in the crankshaft speed. A certain amount of acceleration/deceleration is expected between each firing stroke, but if the crankshaft speed changes are greater than an expected amount, the ECM will interpret this as a misfire. The ECM continuously calculates the crankshaft position from the CKP sensor signals. This information is used in order to determine which cylinder is misfiring so that the ECM can increment the appropriate misfire counter and set the appropriate DTC, if necessary. DTCs P0301 through P0306 correspond to cylinders 1 through 6. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will set. If the misfire is random and on multiple cylinders, DTC P0300 will set.

Conditions for Running the DTC
^ One or more of the following DTCs are not set: P0100, P0115, P0116, P1120, P1220, P0335.
^ The engine speed is between 520 RPM and 6,520 RPM.
^ The Torque Management is not active.
^ The ABS/TC is not active.
^ Rough Road is not detected.
^ The fuel cutoff is not active (including the traction control, the deceleration, the high speed/RPM ).
^ The engine has been running for longer than 5 seconds.
^ The IAT is greater than -8°C (17°F).
^ The EVAP diagnosis is not active.

Conditions for Setting the DTC
^ A misfire that is greater than the legislated limit (amount of misfire ranging from 1.0 - 12.5 percent).
^ The above conditions met for at least 5 camshaft revolutions.

Action Taken When the DTC Sets
^ The ECM will illuminate the Malfunction Indicator Lamp (MIL) under the following conditions:
- The ECM determines that the percentage of misfire is high enough in order to cause catalyst damage. The ECM will flash the Malfunction Indicator Lamp (MIL) during the time that the catalyst damaging misfire is present. The ECM will switch off the injector (maximum of 2) related to the misfiring cylinder, and the MIL will illuminate continuously.
- The ECM determines that the percentage of misfire is high enough in order to cause excessive tail pipe emissions. The ECM illuminates the MIL on the next consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions (the load, the RPM, the temperature.) as the MIL illuminated in the previous ignition cycle.
^ The first drive trip that this diagnostic fails (Fail Pending), this information will store in the Freeze Frame. Any data that was previously stored in the Freeze Frame will be overwritten by the Freeze Frame data for this DTC. The only exception to this is if a Fuel Trim DTC was already recorded in the Freeze Frame. In this case, the Fuel Trim data stays in the Freeze Frame and the Misfire data will not store.

Conditions for Clearing the MIL/DTC

Important: If the last failure was during a non-typical driving condition, the MIL may remain on longer than three drive trips. Review the Freeze Frame for the last failure conditions.

Important: If the fuel cutoff was active as a result of catalyst damaging misfire, and a catalyst damaging misfire no longer exists, the fuel cutoff can only be de-activated by a new ignition cycle.

^ The ECM turns off the MIL after three consecutive drive trips that the diagnostic runs and does not fail within the same conditions that the DTC last failed.
^ A History DTC clear after forty consecutive warm-up cycles if no failures report by this diagnostic or any other emission related diagnostic.
^ The ECM battery voltage is interrupted.
^ Cleared by a scan tool.

Diagnostic Aids
A misfire DTC could be caused by an excessive vibration from sources other than the engine. Check for the following possible sources:
^ Tire/wheel out of round/balance
^ Variable thickness brake rotor/drum
^ Drive shaft not balanced
^ Certain rough road conditions
^ Damaged accessory drive belt

Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. DTCs that may cause a misfire DTC to set may include those for injector control circuits, oxygen sensor, or fuel trim.
7. This test is to determine if there is a problem in the HO2S sensor or wiring that is causing the ECM to alter the fuel trim. When the sensors are disconnected, the fuel system should go into open loop. If the misfire counters stop incrementing, perform the HO2S diagnosis for banks 1 and 2, sensor 1.