Diagnostic Aids

Inspect for any of the following conditions:

• If any DTCs other than misfire (P0300-P0304) are present, diagnose those DTCs first.
• An intermittent ignition system malfunction may cause DTC P0301-P0304 to set. Check ignition system performance with an engine oscilloscope.
• The scan tool data parameters Engine Speed at Misfire and Load at Misfire are fixed at the time the MIL illuminates. These scan tool displays can be used like additional lines of Freeze Frame data. Review the information located in the Engine Speed at Misfire and the Load at Misfire scan tool parameters. The information can help determine engine operation when the misfire occurred.
• Review the misfire counters located in the Engine Data list of the scan tool while the engine is running. If the Total Misfire Current data parameter is increasing, the misfire condition is present. Misfire activity can be monitored with the scan tool in the Misfire Current Cyl # data list parameter of the appropriate cylinder. Use this information in order to determine if the fault is present or an intermittent malfunction.
• An intermittent can also be the result of a defective CKP sensor signal rotor. Remove the CKP sensor and inspect the signal rotor through the sensor hole. Check the condition of the signal rotor for nicks, dents, missing teeth, and foreign material. Refer to Crankshaft Position (CKP) Sensor Replacement .
• Inspect for engine overheating. Refer to Engine Overheating in Engine Cooling.
• A MAF sensor output that is incorrect may cause the PCM to command the fuel system to go lean. Check the performance of the MAF sensor. If the heated oxygen sensor (HO2S) voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition that can cause engine misfire.

An intermittent malfunction may be caused by a fault in the electrical circuits of the ignition system or fuel system of the misfiring cylinder. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.