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.
• ENGINE SPEED AT MISFIRE and LOAD AT MISFIRE scan tool data parameters are fixed at the time MIL illuminates. These 2 scan tool displays can be used like 2 additional lines of freeze frame data. Review ENGINE SPEED AT MISFIRE and LOAD AT MISFIRE scan tool parameters located in scan tool ENGINE DATA list. The information can help determine vehicle conditions when the misfire occurred.
• Review misfire counters located in ENGINE DATA LIST of scan tool while engine is running. If TOTAL MISFIRE CURRENT COUNT data parameter is increasing, the misfire condition is present. A current DTC P0300 should indicate misfire activity in TOTAL MISFIRE CURRENT COUNT parameter. Use this information to determine if fault is present or an intermittent malfunction.
• An intermittent can also be the result of a defective CKP sensor signal rotor. Remove CKP sensor and check the signal rotor through the sensor hole. Check condition of the signal rotor for nicks, dents, missing teeth, and foreign material.
• A Mass Air Flow (MAF) sensor output that is incorrect may cause PCM to command fuel system to go lean. Check performance of the MAF sensor. If HO2S voltage is fixed low, while fuel trim is constantly high, check MAF sensor signal circuit for resistance. High circuit resistance can cause 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 PCV system malfunction. Perform functional check of PCV valve. See POSITIVE CRANKCASE VENTILATION under EMISSION CONTROLS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.

An intermittent malfunction may be caused by a fault in the ignition system or fuel system electrical circuits. Check wiring harness and components for an intermittent condition. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 1.8L VIBE article. Repair any electrical circuit faults that were found. The information included in freeze frame data can be useful in determining the vehicle operating conditions when DTC first set.