GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 2WD 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 DTC P0300

Circuit 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 individual misfire events. A misfire rate that is high enough can cause three-way catalytic converter damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present.

Conditions for Running the DTC
    • DTCs P0013, P0014, P0105, P0107, P0108, P0117, P0118, P0122, P0123, P0125, P0128, P0217, P0218, P0336, P0340, P0341, P0365, P0366, P0502, P0503, P1114, P1115, P1121, P1122, P1336, or P1345 are not set.
    • The engine is running.
    • The system voltage is 10-18 volts.
    • The engine coolant temperature (ECT) is 7-130°C (19-266°F).
    • The throttle angle is steady.
    • The A/C compressor clutch is not changing state.
    • The exhaust gas recirculation (EGR) flow test is inactive.
    • The fuel level is above 10 percent.
    • The fuel cut-off or decel fuel cut-off (DFCO) mode is not active.

Conditions for Setting the DTC
    • The PCM determines that an emission type misfire is present on more than 1 cylinder.
    • The PCM determines that a catalyst damaging misfire is present.

Action Taken When the DTC Sets
    • If the misfire level is non-catalyst damaging, the PCM will illuminate the MIL during the second key cycle in which the DTC sets.
    • If the misfire is severe enough to cause possible catalyst damage, the PCM will immediately flash the MIL while the misfire remains at catalyst damaging levels.
    • The PCM will disable the torque converter clutch (TCC) operation.
    • The PCM will store the conditions present when the DTC set as Freeze Frame/Failure Records data.
        Any data previously stored in Freeze Frame will be overwritten when this DTC sets. The only exception to this is if a fuel trim DTC was already recorded in Freeze Frame. In this case, the fuel trim data stays in Freeze Frame and the Misfire Data will be stored in Failure Records.

Conditions for Clearing the MIL/DTC
    • The PCM will turn the MIL OFF after three consecutive trips during which the diagnostic runs and passes.
        Note that if the last failure was during a non-typical driving condition, the MIL may remain ON longer than three drive trips. Review Freeze Frame for the last failure conditions.
    • The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool Clear DTC Information function.

Diagnostic Aids

Check for the following conditions:

    • Incorrect CKP system variation data stored in the PCM--Run the Crankshaft Position System Variation Learn using the scan tool.
    • System grounds--Ensure that all connections are clean and properly tightened.
    • Damaged accessory drive belt or driven accessory--A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
    • Fuel pressure--Perform a fuel system pressure test. A malfunctioning fuel pump, plugged filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis .
    • Fuel injectors--Refer to Fuel Injector Solenoid Coil Test .
    • Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Extended idle--Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors (HO2S). The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to be set, review the Freeze Frame/Failure Records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80°C (176°F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s.
    • Running the vehicle low on fuel--This DTC may set if the vehicle misfires while running out of fuel. If no misfire is currently present, but the History Counters indicate random misfire was once present, ask if the vehicle has been run out of fuel. If this is the case, no further repair may be necessary.

Important: If the level of misfire was sufficient to cause possible catalyst damage, if the MIL was flashing, ensure that the DTC P0420 test is completed and passed after verifying the misfire repair.

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

  1. Engine misfire can change engine load characteristics which may affect the throttle position (TP) sensor performance diagnostic. Correct any misfire before diagnosing. A malfunctioning injector circuit, crankshaft position system variation not learned, an intermittent CKP sensor problem, incorrect rough road data from the electronic brake control module (EBCM), etc. may cause a misfire DTC to be set. If any other DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.

    2.

  2. The Misfire Bar Graph (or Misfire Current Cyl #) display may normally display a small amount of activity (0-10 counts) but should not steadily increment during an entire 200 revolution test sample period.

    3.

  3. A misfiring cylinder can sometimes affect the misfire counters for other cylinders. When performing this table, concentrating on the cylinder with the highest level of misfire and correcting the problem, may also correct the other cylinders that indicate misfire. This step splits the table between conditions that would cause a specific cylinder to misfire and conditions that would affect all cylinders, causing a random multiple cylinder misfire.

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important:: Incorrect CKP system variation data in the PCM may cause DTC P0300 to set with no apparent misfire. Any of the following conditions can cause incorrect CKP system variation data:

   • Replacing the engine
   • Replacing the PCM
   • Reprogramming the PCM
   • Replacing the CKP sensor
   • Performing internal engine repairs that affect the CKP sensor to crankshaft reluctor wheel relationship
If any of the above conditions apply to this vehicle, the CKP System Variation Learn Procedure must be performed before proceeding with misfire diagnosis.

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Start the engine.
  2. Select the Misfire Bar Graph on the scan tool. The Misfire Current counters can also be used.

Do any cylinders indicate an excessive amount of misfire?

Go to Step 4

Go to Intermittent Conditions

4

Does the misfire appear to be affecting specific cylinders?

Go to Step 8

Go to Step 5

5
  1. Inspect the following system grounds:
    2. • PCM grounds
    • Ignition control module grounds
  2. Ensure that the grounds are clean, tight, and in their proper locations.
  3. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 6

6
  1. Test the fuel pressure. Refer to Fuel System Diagnosis .
  2. If a fuel system pressure condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 7

7
  1. Test for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 8

8
  1. Inspect the following areas for vacuum leaks:
    2. • Vacuum leaks between the intake manifold and the cylinder head
    • Injector O-rings
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

9
  1. Test for proper fuel injector operation. Refer to the following:
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 10

10
  1. Test for proper ignition system operation. Refer to Electronic Ignition (EI) System Diagnosis .
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

11
  1. Inspect and/or test for the following mechanical problems:
    2. • A damaged accessory drive belt or pulley
    • A damaged driven accessory, such as a generator, a water pump, a drive belt a tensioner, etc.
    • A loose or broken motor mount
    • A base engine mechanical problem--Refer to Symptoms - Engine Mechanical in Engine Mechanical.
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

12
  1. Test for proper transaxle TCC operation. Refer to Torque Converter Diagnosis in Automatic Transmission - 460L-E.
  2. If a condition is found, repair as necessary .

Did you find and correct the condition?

Go to Step 13

Go to Intermittent Conditions

13
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 14

Go to Step 2

14

With a scan tool, observe the stored information, Capture Info.

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

Go to Diagnostic Trouble Code (DTC) List

System OK