P0300

SYSTEM DESCRIPTION
The Powertrain Control Module (PCM) uses information from the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause the 3-way catalytic convener to overheat under certain driving conditions. The Malfunction Indicator Lamp (MIL) will flash ON and OFF when the conditions for three-way catalytic converter overheating are present.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0128, P0335, P0336, P0341, P0342, P0343, P0500, P0502, P1114, P1115, P1120, P1220, P1258, or P1336 are not set.
^ The engine speed is between 450-5,000 RPM.
^ The ignition voltage is between 10-18 volts.
^ The Engine Coolant Temperature (ECT) is between -7 and 130°C (19-266°F).
^ The fuel level is more than 10 percent.
^ The Throttle Position (TP) sensor angle is steady within 1 percent.
^ The Antilock Brake System (ABS) and traction control systems are not active.
^ The transmission is not changing gears.
^ The Secondary Air Injection (AIR) diagnostic test is not in progress (RPO NC1 only).
^ The A/C clutch is not changing states.
^ The PCM is not in fuel shut-off or decel fuel cut-off mode.
^ The Anti-lock Brake System (ABS) signal is not exceeding any rough road thresholds.

CONDITIONS FOR SETTING THE DTC
^ The PCM determines that an emission type misfire is present.
^ The PCM determines that a catalyst damaging misfire is present.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. If you are diagnosing or replacing the PCM, inspect the PCM connector gaskets. Verify that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

^ Running the vehicle out of fuel causes sufficient misfire to set DTC P0300. A vehicle that is out of fuel may have fuel level DTCs also set.
^ Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. Component Tests and General Diagnostics
^ A restricted fuel filter can cause sufficient misfire to set DTC P0300. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Excess vibration from sources other than the engine could cause a misfire DTC. The following are possible sources of vibration:
- Variable thickness brake rotor
- Drive shaft not balanced
- Certain rough road conditions
^ If more than one cylinder is misfiring, the scan tool may only display one cylinder misfiring. This will not be apparent until the repair is completed. Also, if an ignition coil ground circuit is open for one side of the engine, the scan tool may only display 2 or 3 cylinders misfiring. Inspect the ground circuit for the ignition coil on the cylinder bank of the engine that has more then one cylinder misfiring.
^ Perform the following procedures to diagnosis conditions that may be causing the misfire counter to increment:
- Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply the water.
- If the Misfire Current counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic test for DTC P0335 first if this condition is suspected. P0335
- If a misfire is present and you suspect a fuel control condition, force the fuel system into open loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, refer to the Scan Tool Data List. Scan Tool Data List
- A misfire may not be apparent at idle. The misfire may only occur above idle under a load. Road test the vehicle and monitor the misfire current counters.
- If more than one cylinder is misfiring, the misfire current counters may only increment for one cylinder. For example, cylinders 1 and 8 are both misfiring, yet only cylinder 8 increments on the misfire current counter.
- If one of the injector fuses is open, only 2 or 3 misfire current counters may increment for the corresponding side of the engine.

TEST DESCRIPTION

Steps 1-3:

Steps 4-7:

Steps 8-19:

The numbers below refer to the step numbers on the diagnostic table.
2. This step diagnoses the conditions causing the misfire counter to increment.
4. The cylinder with the more significant misfire may cause another cylinder counter to increment only by a small amount.
10. If the engine misfire moves with the spark plug, this is good indication that you should replace the spark plug.
12. An Engine condition can cause a spark plug to gas foul. Inspect for loose rockers, collapsed lifters, or worn camshaft lobes.
17. If the customer's concern is the MIL flashing, this indicates that a Catalyst Misfire has occurred. Drive the vehicle in the conditions to run the catalyst diagnostic test.