The engine control module (ECM) uses information from the crankshaft position (CKP) sensor to determine when an engine misfire occurs and uses information from the camshaft position (CMP) sensor 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. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 is set. Under certain driving conditions, a misfire rate can be high enough to cause the 3-way catalytic converter (TWC) to overheat, possibly damaging the converter. The malfunction indicator lamp (MIL) will flash when the three-way catalytic converter overheating, damaging conditions are present and DTC P0300 is set.

The ECM detects a crankshaft rotation speed variation indicating a misfire sufficient to cause emission or three-way catalytic converter damaging levels exceed mandated standards.

Schematic Reference

Electrical Information Reference

DTC Type Reference

Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

J 26792 Spark Tester

1. Verify that no other DTCs are set.

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

2. With the engine idling, verify that there is no abnormal engine noise.

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

3. Observe the Misfire Current counters using the scan tool. The Misfire Current Counters should not be incrementing. Random misfires that are not steadily incriminating may be normal.
4. If the vehicle passes the Circuit/System Verification Test, operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions observed from the Freeze Frame/Failure Records.

Important: You must perform the Circuit/System Verification before proceeding with the Circuit/System Testing.

1. Verify that none of the following conditions exist:

•	An engine vacuum leak

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

•	Fuel Contamination--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, but leave the electrical connector connected.
3. Install the J 26792 to the boot of an ignition coil and ground.

Important: An erratic or weak spark is considered as no spark.

4. Attempt to start the engine and observe J 26792 spark should be a 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 on 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 within the conditions that the misfire occurred. This may include putting the engine under a load, such as turning on the A/C, seat heaters, rear window defog. Observe the Misfire Current counters using the scan tool. The misfire should not follow the spark plug exchange.

⇒	If the misfire is related to the spark plug, replace the spark plug.

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

•	Fuel injector too lean or too rich--Refer to Injector Diagnosis with J39021 or with Tech 2 , Fuel Injector Diagnosis .

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

If the customer concern was a flashing MIL, perform the following procedure:

1. The installed or replaced components or connector 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 DTC.
4. Turn the ignition OFF 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, to verify that 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 procedures.
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 or P0430. Refer to DTC P0420 or P0430 .