The engine control module (ECM) uses information from the crankshaft position (CKP) sensor in order to determine when an engine misfire is occurring. Information from the camshaft position (CMP) sensor is used by the ECM to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. Under certain driving conditions, a misfire rate that is high enough can cause the 3-way catalytic converter (TWC) to overheat, possibly damaging the converter. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present, and DTC P0300 is set.

The ECM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission levels to exceed mandated standards.

DTC P0300 is a Type B DTC.

A misfire may only occur when the engine is under a load or when the engine is cold.

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

Schematic Reference

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Special Tools Required

J 26792 (SAE19992) HEI Spark Tester

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 that no other DTCs are not set.

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

3. Observe the Misfire Current Counters with a scan tool. The Misfire Current Counters should not be incrementing.
4. 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 List.

1. Verify that the following conditions do not exist:

•	Engine 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 .

•	Restricted exhaust system--Refer to Restricted Exhaust .

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

2. Ignition OFF, remove the ignition coil from the misfiring cylinder, leaving the electrical connector connected.
3. Install the J 26792 (SAE19992) to the boot of an ignition coil 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 (SAE19992). 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

•	Cracked, worn, incorrectly gapped--Refer to Spark Plug Inspection .

⇒	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. Operate the engine under the same conditions in which the misfire occurred. Observe the Misfire Current Counters with a scan tool. The misfire should not follow the spark plug exchange.

⇒	If the misfire follows the spark plug, 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 Diagnosis .

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

1. Install any components or connectors that have been removed or replaced during diagnosis.
2. Perform any adjustment, programming, or setup procedures that are required when a component or module is removed or replaced.
3. Clear the DTCs.
4. Turn OFF the ignition for 60 seconds.
5. If the repair was related to a DTC, duplicate the Conditions for running the DTC and use the Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
6. To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420. Refer to DTC P0420 .