P0300

CIRCUIT DESCRIPTION
The PCM has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the ignition control module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring. If 2 percent or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 11 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible three-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.

CONDITIONS FOR RUNNING THE DTC
^ No VSS, CKP, TP, MAP, ECT, CAM or MAF DTCs are set.
^ Engine has been running for more than 5 seconds.
^ Engine speed between 550 and 5800 RPM.
^ System voltage between 9 and 18 volts.
^ The ECT sensor indicates a temperature between -6°C (21°F) and 120°C (248°F).
^ Fuel cutoff is not active.
^ Fuel level is more than 10 percent.
^ Throttle angle is steady.

CONDITIONS FOR SETTING THE DTC
The PCM detects a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels that exceed the mandated standard.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed unless three way converter damage is possible in which case the light will illuminate after the first failure.
^ If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

DIAGNOSTIC AIDS
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 History Cyl #1 - #6) will still contain a value that represents the level of misfire detected on each cylinder. The scan tool displayed misfire counter values (Misfire History Cyl. #1 through #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:

^ Secondary ignition wires.
Inspect the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure less than 30,000 ohms (30K ohms).
^ Damaged or malfunctioning ignition coil.
Inspect for cracks, carbon tracking or other damage. Also test coil secondary resistance. Secondary resistance should be between 5000 ohms and 8000 ohms (5K ohms and 8K ohms).
^ Substitute a known good coil. Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

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

^ Crankshaft Position System Variation.
Refer to CKP System Variation Learn Procedure.
The crankshaft position system variation compensating values are stored in the PCM non-volatile memory after a learn procedure has been performed. If the actual crankshaft position variation is not within the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set. The crankshaft position system variation learn procedure is required when any of the following service procedures have been performed:
- PCM replacement or reprogramming
- Engine replacement
- Crankshaft replacement
- Crankshaft balancer replacement
- Crankshaft position sensor replacement
- Any engine repairs which disturbs crankshaft/harmonic balancer to crankshaft position sensor relationship.
^ System grounds.
Ensure all connections are clean and properly tightened.
^ Mass air flow sensor.
A Mass air flow (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 fail 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.
^ Loss of EBCM/EBTCM Serial Data.
If the PCM stops receiving data from the EBCM/EBTCM, DTC P0300 can set due to a loss of rough road data. Check for stored ABS/TCS DTCs, especially DTCs related to a serial data malfunction. Refer to Self-Diagnostics in ABS/TCS.
^ Air Induction System.
Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Inspect for disconnected or damaged vacuum hoses, incorrectly installed or malfunctioning crankcase ventilation 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 malfunctioning fuel pump, plugged filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition.
Refer to Fuel System Pressure Test. Component Tests and General Diagnostics
^ Fuel injectors
Refer to Fuel Inj Coil Test - ECT Between 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics
Refer to Fuel Inj Coil Test - ECT Outside 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics
^ Contaminated Fuel.
Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
^ EGR System.
Inspect 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 the heated oxygen sensors. 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 be set, review the freeze frame and fail records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures less than 80°C (176°F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows more than 15 gm/s.

IMPORTANT: If the level of misfire was sufficient to cause possible catalyst damage (if the MIL was flashing), ensure that the DTC P0420 test is completed and passed after verifying the misfire repair. Reviewing the Freeze Frame/Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

TEST DESCRIPTION

Steps 1 - 11:

Steps 12 - 17:

Steps 18 - 23:

The numbers below refer to the step numbers on the diagnostic table.
2. A condition that sets a DTC other than P0300 may also cause a misfire. If any of the indicated DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.
3. The Misfire Current Cyl # parameter may normally indicate a small amount of activity (0 - 10 counts) but should not steadily increment during an entire 200 revolution test sample period.
4. Depending on the cause of the misfire, the Misfire History Cyl # counter will indicate a very large number for the misfiring cylinders; values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinders. When investigating a misfire, always start with the components associated with the cylinders that has the largest number of counts stored in the Misfire History Cyl # counter.
5. Steps 5 through 12 test for conditions that can cause a random cylinder misfire.
13. Steps 13 through 22 test for conditions that can cause a non-random or single cylinder misfire.