Diagnostic Aids

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NOTE: If level of misfire was sufficient to cause possible catalyst damage (if MIL was flashing), ensure that DTC P0420 test has been completed and passed after verifying misfire repair. See DTC P0420: CATALYST SYSTEM LOW EFFICIENCY .

Scan tool provides information that can be useful in identifying misfiring cylinder. If DTC P0300 is currently stored as DTC status FAILED SINCE CODE CLEAR, misfire history counters (MISFIRE HIST #1-#6) will still contain a value that represents level of misfire detected on each cylinder.

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

PCM has been replaced.
DTC P1336 is set.
Engine has been replaced.
Crankshaft has been replaced.
Crankshaft harmonic balancer has been replaced.
Crankshaft position sensor has been replaced.

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

Secondary ignition wires. Check secondary wires associated with affected cylinder pair for being disconnected or for excessive resistance (except police 3000 ohms/foot, police 600 ohms/foot).
Damaged or malfunctioning ignition coil. Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be 5000-8000 ohms.

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

If misfire is random, check for the following conditions:

System grounds. Ensure all connections are clean and properly tightened.
Mass Airflow (MAF) sensor. A MAF sensor output that causes PCM to sense a lower than normal airflow will cause a lean condition. Try operating vehicle within fail records conditions with MAF sensor disconnected. If lean or misfiring condition is not present with MAF sensor disconnected, replace MAF sensor.
A damaged accessory drive belt or a belt-driven accessory. A damaged serpentine belt or a belt-driven accessory can cause sufficient engine load variations to set a misfire DTC.
Air induction system. Vacuum leaks that cause intake air to by-pass MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, an incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at throttle body, EGR valve, and intake manifold mounting surfaces.
Fuel pressure. Perform a fuel system pressure test. A malfunctioning fuel pump, plugged filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition.
Fuel injectors.
Water contamination in fuel system can cause a single cylinder to misfire as well as cause a random misfire.
EGR system. Check for 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 heated oxygen sensors. 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 DTC P0300 to set, review FREEZE FRAME/FAILURE RECORDS for DTC P0300. If DTC P0300 occurs at high engine speeds, condition previously described did not cause DTC to set. If DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures less than 176°F (80°C), condition previously described is very likely cause of DTC to set. Deposits on heated oxygen sensors can be eliminated by operating vehicle fully warm at mass airflows more than 15 grams/second.