GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Generic 🢒 2004 🢒 Unit Repair 🢒 DTC 🢒 DTC P0300

System Description

The powertrain control module (PCM) 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 PCM is able to detect misfire events.

Conditions for Running the DTC
    • DTCs P0105, P0107, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0171, P0172, P0336, P0340, P0341, P0502, P0503, P0506, P0507, P0601, P0742, P1133, P1621 are not set.
    • The CNG fuel level is more than 4 percent.
    • The engine speed is between 469-6,400 RPM.
    • The ignition voltage is between 9-17 volts.
    • The engine coolant temperature (ECT) is between -7 and +123°C (+20-+254°F).
    • The engine has been running for more than 30 seconds.
    • The throttle position (TP) has not increased more than 8 percent in 1 second.
    • The TP has not decreased more than 1.5 percent in 1 second.

Conditions for Setting the DTC

The powertrain control module (PCM) detects a deceleration in the crankshaft speed that is characteristic of either an emission type misfire or a catalyst-damaging type misfire.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

The misfire index counts the number of misfires. The scan tool can monitor the misfire index. There is a current misfire counter and a history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring. Use the history misfire counter for misfires that are not currently present.

Many different conditions can cause an intermittent misfire. Inspect the following conditions:

    • Inspect the ignition control (IC) circuit for an intermittent short to ground.
    • Test or inspect the fuel for the following conditions:
       - Inspect for contaminated fuel.
       - Inspect the fuel filter for a restriction. A restricted fuel filter can cause a misfire.
    • Inspect for sticking intake or exhaust valves.
    • Inspect the heated oxygen sensor (HO2S) for abnormal voltage readings.
    • Inspect for a vacuum leak as a possible cause of the engine misfire.

Test Description

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

  1. This step determines if the engine is misfiring while operating on the gasoline fuel system.

    2.

  2. 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.

    3.

  3. This step verifies that the Crankshaft Variation Learn Procedure is performed. A crankshaft variation that is not learned may cause the misfire counters to increment.

    4.

  4. The spark should be consistent when you test the spark at the spark plug wires. The presence of few sparks then nothing indicates no spark.

    5.

  5. A misfiring cylinder may cause more than one misfire counter to increment. Diagnose the fuel injector circuit of the cylinder that displays the highest level of misfire.

    6.

  6. This step verifies that the catalytic convertor is operating correctly.

DTC P0300 - Engine Misfire Detected

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2
  1. Turn ON the ignition, with the engine OFF.
  2. Observe the DTC information with a scan tool.

Are there any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Start the engine.
  2. Clear the DTCs with a scan tool.
  3. Allow the engine to reach operating temperature.
  4. Observe the Misfire Current Counters with a scan tool.

Are any of the Misfire Current Counters incrementing?

--

Go to Step 6

Go to Step 4

4

  1. Force a switch over to gasoline with the scan tool output controls function.
  2. Observe the Misfire Current Counters with a scan tool.

Are any of the Misfire Current Counters incrementing?

--

Go to DTC P0201, P0202, P0203, or P0204

Go to Step 5

5
  1. Observe the Freeze Frame and the Failure records data 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, and the Failure Records data.

Does the DTC fail this ignition cycle?

--

Go to Step 6

Go to Diagnostic Aids

6

Is the engine misfiring?

--

Go to Step 8

Go to Step 7

7

  1. Perform the Crankshaft Variation Learn Procedure. Refer to Crankshaft Position System Variation Learn in Engine Controls in the J Platform Service Manual.
  2. Start the engine.
  3. Observe the Misfire Current Counters with a scan tool.

Are any of the Misfire Current Counters incrementing?

--

Go to Symptoms - Engine Mechanical in Engine Mechanical in the J Platform Service Manual

Go to Step 18

8

Observe the Misfire Current Counters with a scan tool.

Is there only one Misfire Current Counter incrementing?

--

Go to Step 9

Go to Step 14

9

  1. Turn OFF the ignition.
  2. Remove the Alt. Fuel (AF) Relay.
  3. Remove the fuel pump relay. Refer to Relay Replacement in Wiring Systems in the J Platform Service Manual.
  4. Remove the ignition coil housing assembly, keeping the ignition control (IC) module assembly connected to the harness connector. Refer to Ignition Coil Housing Replacement .

    5. Important: Not grounding the IC module housing may cause an erratic spark.

  5. Connect a jumper wire between the top of the IC module and a good ground.
  6. Install the J 36012-A Ignition System Diagnosis Harness.
  7. Install a J 26792 Spark Tester on the #1 spark plug jumper wire.
  8. Ground the #4 spark plug jumper wire. The #4 wire is the companion to #1.
  9. Crank the engine with the remaining spark plug wires connected.
  10. Repeat the above steps by installing the spark tester on #4 and grounding #1. Do the same for the #2 and the #3 spark plugs. Ensure the companion wire is grounded.

Does the spark tester spark on all cylinders?

--

Go to Step 10

Go to Electronic Ignition (EI) System Diagnosis in Engine Controls in the J Platform Service Manual

10
  1. Remove the spark plug from the cylinder that indicates a misfire. Refer to Spark Plug Replacement in Engine Controls in the J Platform Service Manual.

    2. Important: If the spark plug is gas, coolant, or oil fouled, determine the cause of the fouling. Refer to Spark Plug Inspection in Engine Controls in the J Platform Service Manual for spark plugs that are gasoline fouled. Refer to Symptoms - Engine Mechanical in Engine Mechanical in the J Platform Service Manual for spark plugs that are coolant or oil fouled.

  2. Inspect the spark plug. Refer to Spark Plug Inspection in Engine Controls in the J Platform Service Manual.

Did you find and correct the condition?

--

Go to Step 16

Go to Step 11

11
  1. Swap the suspected spark plug with another cylinder that is operating correctly.
  2. Start the engine.
  3. Operate the engine within the conditions that the misfire occurred.
  4. Monitor the Misfire Current Counters with a scan tool.

Did the misfire move with the spark plug?

--

Go to Step 15

Go to Step 12

12

Important: 

   • Make sure all the injectors operate. High resistance in an injector circuit causes the injector to be inoperative without setting a fuel injector DTC.
   • Return to this diagnostic after you complete the Fuel Injector Coil Test.

Perform the Fuel Injector Coil Test. Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct the condition?

--

Go to Step 16

Go to Step 13

13

Perform the Fuel System Diagnosis. Refer to Fuel System Diagnosis .

Did you find and correct the condition?

--

Go to Step 16

Go to Symptoms - Engine Mechanical in Engine Mechanical in the J Platform Service Manual

14

Inspect or test for the following conditions:

    • Inspect the vacuum hoses for splits, kinks, and the correct connections.
    • Inspect for vacuum leaks at the throttle body and at the intake manifold.
    • Inspect the PCM grounds for being clean, tight, and in the correct location.
    • Test for the correct fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for fouled or damaged spark plugs. Refer to Spark Plug Inspection in Engine Controls in the J Platform Service Manual.
    • Inspect the exhaust system for restrictions. Refer to Restricted Exhaust in Engine Exhaust in the J Platform Service Manual.

Did you find and correct the condition?

--

Go to Step 16

Go to Symptoms - Engine Mechanical in Engine Mechanical in the J Platform Service Manual

15

Replace the spark plug. Refer to Spark Plug Inspection in Engine Controls in the J Platform Service Manual.

Did you complete the replacement?

--

Go to Step 16

--

16

Was the customers concern that the was MIL flashing?

--

Go to Step 17

Go to Step 18

17

  1. Operate the vehicle at the specified value for 4 minutes.
  2. Operate the vehicle within the Conditions for Running the DTC P0420 as specified in the supporting text. Refer to DTC P0420 .

Does the DTC run and pass?

2,500 RPM

Go to Step 18

Go to DTC P0420 in Engine Controls in the J Platform Service Manual

18
  1. Use the scan tool in order to clear the DTCs.
  2. Turn the ignition OFF for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC run and pass?

--

Go to Step 19

Go to Step 2

19

With a scan tool, select Capture Info in order to observe the stored information.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK