GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The diagnostic illuminates the malfunction indicator lamp (MIL) when a non-cylinder specific misfire is present. The powertrain control module (PCM) uses the crankshaft position (CKP) sensor and camshaft position (CMP) sensor to determine engine misfire. The CKP sensor and the CMP sensor monitor their respective components and evaluate changes in the crankshaft rotational speed for each cylinder. Irregular changes in the crankshaft rotational speed indicate a possible misfire. The MIL illuminates when the misfire rate equals or exceeds a pre-determined count. A misfire rate that is high enough can cause the catalytic converter to overheat under certain driving conditions. The MIL will flash ON and OFF when the conditions for catalytic converter overheating are present. Each cylinder is monitored individually for a misfire condition. A DTC P0300 indicates that engine misfire was indicated in more than one cylinder at the same time.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0300 Engine Misfire Detected

Conditions for Running the DTC

    • DTCs P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0222, P0223, P0335, P0336, P1128, P1129, P2227, P2228, and P2229 are not set.
    • The engine speed is between 500-6,500 RPM.
    • The engine coolant temperature (ECT) is more than -10°C (14°F) at start-up.
    • The intake air temperature (IAT) sensor 2 is more than -10°C (14°F) at start-up.
    • The manifold absolute pressure (MAP) is within a specified range for a calculated engine load.
    • DTC P0300 runs continuously once the above conditions are met.

Conditions for Setting the DTC

Condition 1

The PCM detects 30-100 misfires every 200 engine revolutions.

Condition 2

The PCM detects 70 misfires every 1,000 engine revolutions.

Action Taken When the DTC Sets

Condition 1

    • The PCM flashes the MIL the first time the diagnostic fails indicating that catalytic converter damage can occur.
    • The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame data.

Condition 2

    • The PCM illuminates the MIL the second time the diagnostic fails.
    • The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
    • The DTC will clear if there is no misfire detected when the vehicle is operated in the same condition that the misfire was originally set.
    • The DTC can be cleared by using the scan tool Clear DTC Information function.

Diagnostic Aids

Inspect for any of the following conditions:

    • If any DTCs other than misfire P0301-P0306 are set, diagnose those DTCs first.
    • An intermittent ignition system malfunction may cause DTC P0300 to set. Inspect the ignition system performance with an engine oscilloscope.
    • Review the misfire counters located in the Engine Data list of the scan tool while the engine is running. If the Misfire Current Cylinder# data parameter is increasing, the misfire condition is present. Misfire activity can be monitored with the scan tool in the Misfire Current Cyl # data list parameter of the appropriate cylinder. Use this information in order to determine if the fault is present or an intermittent malfunction.
    • Inspect for good quality recently purchased fuel. Inferior quality fuel or fuel that has aged can cause poor combustion that results in detectable engine misfire. If the fuel is suspect, drain and replace as necessary.
    • An intermittent can also be the result of a defective CKP sensor signal rotor. Inspect the CKP signal rotor for nicks, dents, missing teeth, and foreign material.
    • Inspect for engine overheating. Refer to Engine Overheating .

An intermittent malfunction may be caused by a fault in the electrical circuits of the ignition system or fuel system of the misfiring cylinder. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs .

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. The Diagnostic System Check - Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

  2. This step verifies whether the misfire is present. The scan tool will display increasing counts in one or more of the Misfire Current Cyl. # parameter of the misfiring cylinder when a misfire is occurring.

  3. If the Misfire Current Counters are incrementing, but the engine is NOT misfiring, this indicates a mechanical condition. For example, an accessory drive belt could cause this condition.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views or Splice Pack Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2

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

  1. Turn ON the ignition, with the engine OFF.
  2. Clear the DTCs with a scan tool.
  3. Start the engine and allow the engine to idle.
  4. Allow the engine to reach operating temperature.
  5. Monitor the Misfire Current Cyl. 1-6 Counter parameters with a scan tool.

Are any of the Misfire Current Counters incrementing?

--

Go to Step 4

Go to Step 3

3

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

Is the engine misfiring?

--

Go to Step 5

Go to Symptoms - Engine Mechanical

5

Observe the DTC Info with a scan tool.

Is DTC P0201, P0202, P0203, P0204, P0205, P0206, P0335, P0336, P0401, P0403, P0404, P0406, P2647, P2648, or P2649 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 6

6

Is there an engine mechanical noise?

 

Go to Symptoms - Engine Mechanical

Go to Step 7

7

Is there more than one misfire DTC set?

 

Go to Step 8

Go to DTC P0301-P0306

8

  1. Inspect or test for the following conditions:
  2. • Inspect the vacuum hoses for splits, kinks, and proper connections.
    • Inspect the throttle body and the intake manifold for vacuum leaks.
    • Inspect the crankcase ventilation valve and/or system for any vacuum leaks. Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • Test for the correct fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect the fuel system for any restrictions, leaks or fuel contamination. Refer to Fuel System Diagnosis or Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect for fouled or damaged spark plugs. Determine what caused the spark plugs to foul. Refer to Spark Plug Inspection and Spark Plug Replacement .
    • Inspect the exhaust system for restrictions. Refer to Restricted Exhaust .
    • Inspect the splice pack for the ignition 1 circuits of the ignition coils for water intrusion and poor connections. Refer to Testing for Intermittent Conditions and Poor Connections .
    • Inspect the engine control grounds for being clean, tight, and in the correct location.
  3. Repair as necessary.

Did you find and correct the condition?

 

Go to Step 9

Go to Symptoms - Engine Mechanical

9

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 10

10

In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the conditions for running and setting DTC P0420/0430 and verify that DTC P0420/0430 runs and passes. Refer to DTC P0420 or P0430 .

Does the DTC run and pass?

--

Go to Step 11

Go to DTC P0420 or P0430

11

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK