For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provide an overview of each diagnostic category.

### DTC Descriptors

DTC P0300: Engine Misfire Detected
DTC P0301: Cylinder 1 Misfire Detected
DTC P0302: Cylinder 2 Misfire Detected
DTC P0303: Cylinder 3 Misfire Detected
DTC P0304: Cylinder 4 Misfire Detected
DTC P0305: Cylinder 5 Misfire Detected
DTC P0306: Cylinder 6 Misfire Detected

### Circuit/System Description

The engine control module (ECM) 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 ECM is able to detect individual misfire events. A misfire rate that is high enough can cause 3-way catalytic converter damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present.

### Conditions for Running the DTC

• | DTCs P0016, P0102, P0103, P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, and P0340 are not set.

• | The engine speed is less than 4,500 RPM.

• | The engine load is less than 71 percent.

• | The DTCs run continuously when the above conditions are met.

### Conditions for Setting the DTC

The ECM detects a crankshaft rotation speed variation indicating a misfire rate sufficient to cause emissions levels to exceed a predetermined value.

### Action Taken When the DTC Sets

DTCs P0300, P0301, P0302, P0303, P0304, P0305 and P0306 are Type A DTCs.

### Conditions for Clearing the DTC

DTCs P0300, P0301, P0302, P0303, P0304, P0305 and P0306 are Type A DTCs.

### Diagnostic Aids

A misfire DTC could be caused by an excessive vibration from sources other than the engine. Inspect for the following possible sources:

• | A tire or wheel that is out of round or out of balance

• | Variable thickness brake rotors

• | An unbalanced drive shaft

• | Certain rough road conditions

• | A damaged accessory drive component or belt

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls Connector End Views

#### Description and Operation

Electronic Ignition (EI) System Description

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Engine idling at the normal operating temperature. Verify there is no abnormal engine noise.

⇒ | If there is an abnormal engine noise, refer to Symptoms--Engine Mechanical.

2. Verify the following DTCs are not set: P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, or P0277.

⇒ | If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

3. Clear the DTCs with a scan tool.

Important: A misfire may occur when the engine is under a load. An engine load may be necessary to verify the condition.

4. Increase the engine speed to 1,500 RPM, observe the DTC information parameter. DTCs P0300, P0301, P0302, P0303, P0304, P0305, or P0306 should not set.
5. If the vehicle passes the Circuit/System Verification test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

### Circuit/System Testing

1. Verify the following conditions do not exist:

• | Vacuum hose splits, kinks, and incorrect connections

• | Engine vacuum leaks

• | Crankcase ventilation system for vacuum leaks

• | Fuel pressure that is too low or too high--Refer to Fuel System Diagnosis .

• | Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | Exhaust system restrictions--Refer to Restricted Exhaust .

⇒ | If you find any of the above conditions, repair as necessary.

2. Ignition OFF, disconnect the ignition coil from the misfiring cylinder.
3. Install the J 26792 to the boot of the ignition coil and ground.

Important: An erratic or weak spark is considered a no spark condition.

4. Attempt to start the engine. Observe the J 26792 . The spark tester should spark.

⇒ | If there is no spark, refer to Electronic Ignition (EI) System Diagnosis for diagnosis of the ignition coil.

5. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the following conditions do not exist with the spark plug:

• | Gas, coolant, or oil fouled--Refer to Spark Plug Inspection .

• | Cracked, worn, incorrectly gapped

⇒ | If there is a condition with the spark plug, replace the spark plug.

6. Exchange the suspected spark plug with another cylinder that is operating correctly.
7. Engine idling, disconnect the injector that corresponds to the misfiring cylinder. The engine speed should change.

⇒ | If the engine speed changes, replace the spark plug.

8. If all conditions test normal, test or inspect for the following:

• | A lean or rich fuel injector--Refer to Fuel Injector Balance Test with Special Tool .

• | An engine mechanical condition Refer to Symptoms--Engine Mechanical.

### Repair Instructions

• | Spark Plug Inspection

• | Spark Plug Replacement

### Repair Verification

1. If the customer concern was a flashing MIL, operate the vehicle within the conditions for running DTC P0420 or P0430. Refer to DTC P0420 or P0430 .
2. Clear the DTCs with a scan tool.
3. Turn OFF the ignition for 30 seconds.
4. Start the engine.
5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
6. If the DTC fails this ignition, a misfire still exists.