P0300

DTC P0300 Engine Misfire Detected

Refer to Engine Controls Schematics. Electrical Diagrams





















Circuit Description
The PCM has the ability to detect a misfire by monitoring the 3X reference from the ignition control (IC) module and camshaft position input signals from the camshaft position (CMP) sensor. 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 22 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 3-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
* TP, MAP, ECT, CKP, CMP, MAF sensor, VSS, DTCs are not set.
* Engine speed is between 525 and 5900 RPM.
* System voltage is between 9.0 and 18.0 volts.
* The ECT indicates an engine temperature between 70°C (21°F) and 1250°C (253°F).
* Throttle angle is steady.

Conditions for Setting the DTC
The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause 3-way catalytic converter damage or emissions levels to exceed mandated standard.

Action Taken When the DTC Sets
* The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
* The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

Conditions for Clearing the MIL/DTC
* The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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 Test failed since code clear, the misfire history counters (Misfire History numbers 1 and 6) will still contain a value that represents the level of misfire for each cylinder.

* A misfire DTC may set if components that affect the crankshaft position sensor have recently been replaced, and the Crankshaft Position System Variation Learn has not been performed. If the diagnostic table does not identify a problem then perform the Crankshaft Position System Variation Learn. Programming and Relearning
The Crankshaft Position 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 sensor has been replaced.

* The scan tool displayed misfire counter values (Misfire History numbers 1 through 6) can be useful in determining whether the misfire is isolated to a single cylinder or to a cylinder pair, cylinders that share an ignition coil 1 and 4, 2 and 5, 3 and 6. If the largest amount of activity is isolated to a cylinder pair, inspect the following components:
- Secondary Ignition Wires - Inspect wires for affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure between 600 Ohms per foot (1968 Ohms per meter).
- Damaged Or Faulty Ignition Coil - Inspect for cracks, carbon tracking or other damage. Substitute a known good coil with current coil and retest. If the misfire follows the coil, replace the ignition coil.
- Secondary resistance of coil - Secondary resistance should be between 5000-8000 Ohms (5K-8K Ohms).

* If the misfire is random, inspect the following components and conditions:
- System Grounds - Ensure all connections are clean and properly tightened.
- MAF - A mass air flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.
- Air Induction System - Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Inspect for the following conditions:
Disconnected or damaged vacuum hoses
Incorrectly installed or faulty crankcase ventilation valve
Vacuum leaks at the throttle body
Vacuum leaks at the EGR valve
Vacuum leaks at the intake manifold mounting surfaces

- Fuel Pressure - Perform a fuel system pressure test. Inspect for the following conditions that may contribute to a lean condition:
A faulty fuel pump
A plugged filter
A faulty fuel system pressure regulator

- Injectors - Perform injector coil balance test to locate faulty injectors contributing to a lean or flooding condition. In addition to the above test, inspect the condition of the injector O-rings.
- EGR - Inspect for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
- 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. Alcohol In Fuel Testing Procedure

* Other conditions that may contribute to an engine misfire include the following:
- Incorrect engine oil pressure
- Damaged accessory drive belt or pulley
- Damaged driven accessory
- Damaged generator
- Damaged water pump
- Damaged drive belt tensioner
- Loose or broken motor mounts

* Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

* Loose terminal connection
- Use a corresponding mating terminal to test for proper tension.
Refer to Testing for Intermittent Conditions and Poor Connections. Testing For Intermittent and Poor Connections
Refer to Connector Repairs for diagnosis and repair. Connector Repairs
- Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection.
Refer to Testing for Intermittent Conditions and Poor Connections. Testing For Intermittent and Poor Connections
Refer to Connector Repairs for diagnosis and repair. Connector Repairs

* Damaged harness - Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs for diagnosis and repair. Wiring Repairs

* Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs for diagnosis and repair. Wiring Repairs

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

Test Description
The numbers below refer to the step numbers on the diagnostic table:
2. A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the EBCM may cause a misfire DTC to be set. 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 Cylinder number display may normally display 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 Cylinder number counter will display 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 items associated with the cylinders that has the largest number of counts stored in the Misfire History Cylinder number counter.

5. If the misfiring cylinders are companion cylinders, the condition is most likely linked to the ignition system.

9. Inspect for poor terminal connection, grooves, corrosion, pitting, loose fit.

17. Inspect for conditions that may contribute to the engine misfire. Refer to Diagnostic Aids.