Diagnostic Aids

Check for the following conditions:

• If any DTCs other than misfire (P0300 to P0304) are present, diagnose those DTCs first.
• If DTCs P0301, P0302, P0303, or P0304 are also present with DTC P0300, diagnose those DTCs first.
• An intermittent ignition system malfunction may cause DTC P0300 to set. Check the ignition system performance with an engine oscilloscope.
• The engine speed at Misfire and the Load at Misfire scan tool data parameters are fixed at the time the MIL illuminates. These 2 scan tool displays can be used like 2 additional lines of Freeze Frame data. Review the engine speed at Misfire and the Load at Misfire scan tool parameters located on the scan tool Engine Data list. The information can help determine the vehicle conditions 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 Count data parameter is increasing, the misfire condition is present. A current DTC P0300 should indicate misfire activity in the Total Misfire Current Count parameter. 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 .
• 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 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.
• Check for engine overheating.
• Check for a positive crankcase ventilation (PCV) system malfunction. Perform a functional check of the PCV valve. Refer to Crankcase Ventilation System Inspection/Diagnosis .

An intermittent malfunction may be caused by a fault in the ignition system or fuel system electrical circuits. 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.