Diagnostic Aids

WARNING: This page is about a different car, the 2002 Pontiac Firebird and 2002 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.

The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire Hist #1-#6) will still contain a value that represents the level of misfire detected on each cylinder.

A misfire DTC may set if components that affect the Crankshaft Position (CKP) sensor have recently been replaced, and the CKP system variation learn procedure has not been performed. If the diagnostic procedure does not identify a condition, perform the CKP System Variation Learn Procedure. See CRANKSHAFT POSITION SENSOR under PROGRAMMING. The crankshaft position system variation learn procedure should be performed if any of the following conditions are true:

The PCM has been replaced.
DTC P1336 is set.
The engine has been replaced.
The crankshaft has been replaced.
The crankshaft harmonic balancer has been replaced.
The Crankshaft Position (CKP) sensor has been replaced.

The scan tool displayed misfire counter values (Misfire Hist. #1-#6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil 1/4, 2/5, 3/6), or is random. If the largest amount of activity is isolated to a cylinder pair, inspect for the following conditions:

Secondary ignition wires. Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure under 600 ohms per/ft.
Damaged or malfunctioning ignition coil. Check for cracks, for carbon tracking, or for other damage. Also check coil secondary resistance. Secondary resistance should be between 5000-8000 ohms.

Switch the ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

NOTE: If the level of misfire was sufficient to cause possible catalyst damage, and if the MIL was flashing, ensure that the DTC P0420 test is completed and passed after verifying the misfire repair. See DTC P0420: TWC SYSTEM - LOW EFFICIENCY .

If the misfire is random, check for the following conditions:

System grounds. Ensure that all connections are clean and properly tightened.
The Mass Airflow (MAF) sensor. A MAF sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the Failure Records conditions, with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor. See appropriate REMOVAL, OVERHAUL & INSTALLATION article.
A damaged accessory drive belt or driven accessory. A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
The air induction system. Vacuum leaks that cause intake air to by-pass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, for an incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces.
Fuel pressure. Perform a fuel system pressure test. A malfunctioning fuel pump, a plugged filter, or a malfunctioning fuel system pressure regulator will contribute to a lean condition. See appropriate BASIC DIAGNOSTIC PROCEDURES article.
Fuel injectors. Perform fuel injector coil test. See appropriate SYSTEM & COMPONENT TESTING article.
Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire.
The Exhaust Gas Recirculation (EGR) system. Check for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
Extended idle. Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the Heated Oxygen Sensors (HO2S). The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 176°F (80°C), the condition described above is very likely the cause of the DTC to set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s.