P0304

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Steps 3 - 7:

Steps 8 - 13:

Steps 14 - 15:

DTC P0304 Random/Multiple Cylinder Misfire Detected




Circuit Description
If the PCM determines that the engine is misfiring and can determine the actual cylinder that is misfiring then it will log the individual trouble code for that cylinder P0304. This is normally the situation in the case of a failed fuel injector, spark plug or ignition lead

Conditions for Setting the DTC
- None of the following Diagnostic Trouble Codes are present: TP sensor, MAP sensor, CMP sensor, VSS, ECT, CKP sensor.
- The engine speed is between 600 and 6250 RPM.
- The system voltage is between 11 and 16 volts.
- The engine coolant temperature sensor (ECT) indicates an engine temperature between -6.75 °C (20 °F) and 120 °C (248 °F).
- Throttle angle is steady and throttle angle changes less than 2.73% per 100 milliseconds.
- The PCM detects a crankshaft RPM variation indicating a misfire that is sufficient to cause catalytic converter damage or emissions levels to exceed the mandated standard.
- TP sensor reads less than 3.125%.
- Vehicle speed is greater than 33 kph (20 mph).
- Engine under load.

Action Taken When the DTC Sets
- The PCM will illuminate the malfunction indicator lamp (MIL) the first time the fault is detected.
- If the misfire is severe enough to cause possible catalyst damage, the PCM will flash the MIL for as long as the misfire remains at catalyst damaging levels.
- "Open Loop" fuel control will be in effect.
- The PCM will store conditions which were present when the Diagnostic Trouble Code was set as Freeze Frame and in the Failure Records data.

Conditions for Clearing the MIL/DTC
- The PCM will turn the MIL OFF on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
- A history Diagnostic Trouble Code P0304 will clear after 40 consecutive warm-up cycles have occurred without a fault.
- Diagnostic Trouble Code P0304 can be cleared by using the Scan Tool's "Clear Info" function.

Diagnostic Aids
The Tech 2 displays "Misfire Cur. # 1 through # 4" can be useful to determine whether the misfire is isolated to a single cylinder.
- Damaged or faulty ignition coils - Check for cracks or other damage.
- Substitute a known good coil - Swap the ignition coils and retest. If misfire follows the coil, replace the ignition coil.

If the misfire is random, check for the following conditions:
- System grounds - Ensure all connections are clean and properly tightened.
- Air induction system - Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty PCV valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
- Fuel pressure - Perform a fuel system pressure test. A faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.
- Excessive engine vibration - This may falsely set a P0304. Refer to Engine Mechanical Diagnosis to check for a falsely Mechanical condition or component.
- Injector(s) - Perform an injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O-rings.
- EGR - Check for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the Diagnostic Trouble Code to be set occurs. This may assist in diagnosing the condition.