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.

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

DTC P0300 is a Type A (MIL Flashing) and B DTC.

DTC P0300 is a Type A (MIL Flashing) and B DTC.

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

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Special Tools Required

J 26792 Spark Tester

1. Engine running 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. Observe the DTC information with a scan tool. DTCs P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0335, P0336, P0337, P0351, or P0352 should not set.

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

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

3. Engine running, disconnect one fuel injector connector at a time to isolate the misfiring cylinder. The engine speed should change when an injector is disconnected.
4. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

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

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

2. Ignition OFF, disconnect the spark plug wire from the misfiring cylinder.
3. Install the J 26792 Spark Tester between the boot of the spark plug wire and ground.

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

4. Attempt to start the engine and observe the J 26792 Spark Tester. The spark tester should spark.

⇒	If there is no spark, test the spark plug wire for less than 1,000 ohms per 30 cm (per foot). Replace the spark plug wire if more than the specified value. If the spark plug wire test normal, 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 running, disconnect the injector that corresponds to the misfiring cylinder. The engine speed should change.

⇒	If the engine speed changes, replace or re-gap 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

1. If the customer concern was a flashing MIL, operate the vehicle within the conditions for running DTC P0420. Refer to DTC P0420.

⇒	If DTC P0420 does not run and pass, diagnose that DTC.

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.