Diagnostic Aids

WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

Check for any of the following conditions:

If any DTCs other than misfire (P0300 to 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 MIL illuminates. These scan tool displays can be used like additional lines of freeze frame data. Review information located in ENGINE SPEED AT MISFIRE and LOAD AT MISFIRE scan tool parameters. Information can help determine engine operation when misfire occurred.
Review misfire counters located in ENGINE DATA list of scan tool, while engine is running. If TOTAL MISFIRE CURRENT data parameter is increasing, misfire condition is present. Misfire activity can be monitored using scan tool in MISFIRE CURRENT CYL NO. data list parameter of appropriate cylinder. 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 signal rotor for nicks, dents, missing teeth, and foreign material.
Check for engine overheating.
A Mass Air Flow (MAF) sensor output that is incorrect may cause PCM to command fuel system to go lean. Check performance of MAF sensor. If Heated Oxygen Sensor (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.

An intermittent malfunction may be caused by a fault in the electrical circuits of ignition system or fuel system of misfiring cylinder. 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. Information included in freeze frame data can be useful in determining vehicle operating conditions when DTC first set.