The engine control module (ECM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to detect which cylinder is causing a misfire. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events which can be viewed on a scan tool.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

Electrical Information Reference

Scan Tool Reference

Engine Control Module Scan Tool Information

Special Tools

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, repair as necessary.

2. Observe the DTC information with a scan tool. Verify that DTC P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0340, P0341, P0335, P0336, P0351, P0352, P0353, P0354, P0355, P0356, P0357, or P0358 is not set.

⇒	If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List .

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

3. Observe the scan tool Misfire Current Counter parameters. The Misfire Current Counters should not be incrementing.

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

•	Restricted exhaust system

⇒	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 to the boot of a 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 . The spark tester should spark.

⇒	If there is no spark, test the spark plug wire for the correct resistance. Refer to Spark Plug Wire Replacement . Replace the spark plug wire if necessary. 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 the following conditions do not exist with the spark plug:

•	Gas, coolant, or oil fouled.

•	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, observe the scan tool Misfire Current Counters. 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 Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2

•	An engine mechanical condition

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.