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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2002 🢒 Regal 🢒 Body and Accessories 🢒 Body Control System 🢒 DTC P0300

Circuit Description

The powertrain control module (PCM) uses information from the ignition control (IC) module and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring the 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 the 3-way catalytic converter (TWC) to overheat under certain conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0107, P0108, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0336, P0341, P0502, P0503, P1106, P1107, P1114, P1115, P1121, P1122, P1336, or P1374 are not set.
    • The engine speed is between 550-5,850 RPM.
    • The system voltage is between 9-18 volts.
    • The engine coolant temperature (ECT) indicates an engine temperature between -6°C (21°F) and 120°C (248°F).
    • The throttle angle is steady.

Conditions for Setting the DTC

The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause TWC damage or emissions levels to exceed mandated standard.

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 scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire Hist #1-#6) will still contain a value that represents the level of misfire detected on each cylinder.
    • A misfire DTC may set if components that affect the crankshaft position (CKP) sensor have recently been replaced, and the CKP system variation learn procedure has not been performed. If the diagnostic table does not identify a condition then perform the Crankshaft Position System Variation Learn . The CKP system variation learn procedure should be performed if any of the following conditions are true:
       - The PCM has been replaced.
       - DTC P1336 is set.
       - The engine has been replaced.
       - The crankshaft has been replaced.
       - The crankshaft harmonic balancer has been replaced.
       - The CKP sensor has been replaced.
    • The scan tool displayed misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair, cylinders that share an ignition coil, such as 1/4, 2/5, 3/6, or is random. If the largest amount of activity is isolated to a cylinder pair, inspect for the following conditions:
       - The secondary ignition wires--Inspect the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure less than 600 ohms per foot.
       - A damaged or malfunctioning ignition coil--inspect for cracks, for carbon tracking, or for other damage. Also inspect the coils secondary resistance. The secondary resistance should be between 5,000-8,000 ohms (5-8K ohms).
    • Switch the ignition coils, and re-test the coils. If the misfire follows the coil, replace the ignition coil.

       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 misfire is random, inspect for the following conditions:
       - The system grounds--Confirm all connections are clean and properly tightened.
       - The mass air flow sensor (MAF).
       - A MAF sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the failure records conditions with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the Mass Airflow Sensor Replacement .
       - A 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.
       - The air induction system--Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Inspect for disconnected or damaged vacuum hoses, for incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the exhaust gas recirculation (EGR) valve, and the intake manifold mounting surfaces.
       - The 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 .
       - The fuel injectors
       - Refer to Fuel Injector Solenoid Coil Test
       - Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
       - The EGR system--Inspect for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or cause excessive EGR flow.
       - An 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 the HO2S 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 set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at ECT below 80°C (176°F), the condition described above is very likely the cause of the DTC to set. The deposits on the HO2S can be eliminated by operating the vehicle fully warm at MAF above 15 g/s.

Test Description

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

  1. A malfunctioning injector circuit, CKP system variation not learned condition, or incorrect rough road data from the electronic brake control module (EBCM) may cause a misfire DTC to be set. If any of the indicated DTCs are set with DTC P0300, diagnose and repair the other DTC before using the DTC P0300 table.

    2.

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

    3.

  3. Depending on the cause of the misfire, the Misfire History Cyl # counter will display a very large number for the misfiring cylinders. Values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinders. When investigating a misfire, always start with items associated with the cylinders that have the largest number of counts stored in the Misfire History Cyl # counter.

    4.

  4. Steps 5-12 test for conditions that can cause a random cylinder misfire.

    5.

  5. Steps 13-23 test for conditions that can cause a non-random or single cylinder misfire.

DTC P0300 - Engine Misfire Detected

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Start and idle the engine.
  2. Review and record the scan tool Freeze Frame data.
  3. Operate the vehicle within the conditions present when the DTC was set.
  4. Monitor the scan tool Misfire Current Cyl # display for each cylinder.

Is Misfire Current # display increments for any cylinder, indicating a misfire currently occurring?

--

Go to Step 4

Go to Diagnostic Aids

4

View the Misfire History Cyl # display with a scan tool.

Does Misfire History Cyl # display a very large value for more than one cylinder?

--

Go to Step 5

Go to Step 13

5

Visually and physically inspect the vacuum hoses for splits, for kinks, and for improper connections. Refer to Emission Hose Routing Diagram . If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 6

6

Inspect the crankcase ventilation valve for improper installation and for damaged O-rings. If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 7

7

Inspect the throttle body inlet screen for damage or for the presence of foreign objects that may alter the air flow sample through the MAF sensor.

If a condition is found, repair as necessary. Refer to Throttle Body Air Inlet Screen Replacement .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 8

8

Test fuel pressure. Refer to Fuel System Diagnosis . If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 9

9

Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . If a condition was found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 5

Go to Step 10

10

  1. Visually and physically inspect the PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations. Refer to Ground Distribution Schematics in Wiring Systems.
  2. If a condition is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 11

11

  1. Inspect the following areas for vacuum leaks:
    2. • The intake manifold
    • The EGR adapter
    • The EGR valve
    • The EGR feed pipes
    • The injector O-rings
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 12

12

  1. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .
  2. Visually and physically inspect the valve to ensure that the pintle is not sticking partially open. Also, inspect the EGR valve pintle and seat for carbon deposits or burrs that may interfere with the pintle closing completely.
  3. If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 13

13

  1. Test for proper fuel injector operation. Refer to Fuel Injector Solenoid Coil Test .
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 14

14

  1. Visually and physically inspect the ignition wires associated with the cylinders which were misfiring. Refer to Spark Plug Wire Inspection .
  2. If a condition is found, repair as necessary. Refer to Spark Plug Wire Replacement .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 15

15

  1. Install a J 26792 Spark Tester at the spark plug end of the ignition wire for the cylinder that is indicated by the Misfire Current Cyl # counters or by the Misfire History Cyl # counters as having the most severe misfire, indicated by the largest number of counts.
  2. Jumper the spark plug end of the companion cylinder ignition wire to engine ground.

    3. The companion cylinder is the cylinder that shares the same ignition coil (i.e., 1/4; 2/5; 3/6).

  3. Crank the engine while observing the J 26792 . A spark should be observed.

Is spark present?

--

Go to Step 20

Go to Step 16

16

  1. Remove and inspect the ignition wires associated with the cylinders that were indicated as misfiring. Refer to Spark Plug Wire Inspection . Ensure that the wires and boots are free of carbon tracking, and that the insulation is not damaged.
  2. If a condition is found, repair as necessary. Refer to Spark Plug Wire Replacement .

    3. Important: If carbon tracking or terminal discoloration is apparent at the ignition coil end of any of the ignition wires, replace the affected ignition wire and the associated ignition coil. Refer to Ignition Coil Replacement .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 17

17

Measure the resistance of the spark plug wires associated with the cylinders that were indicated as misfiring. Refer to Ignition System Specifications .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 18

18

  1. Remove and inspect the ignition coils associated with the cylinders that were indicated as misfiring. Ensure that the coils and the coil towers are free of cracks and carbon tracking.
  2. If a condition is found, repair as necessary. Refer to Ignition Coil Replacement .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 19

19

  1. Measure the ignition coil secondary resistance.
  2. If resistance is not between the specified values, replace the malfunctioning ignition coils as necessary. Refer to Ignition Coil Replacement .

Did you find and correct the condition?

5-8K ohms

(5,000-8,000 ohms)

Go to Step 25

Go to Step 24

20

  1. Remove the spark plugs from the cylinders that were indicated as misfiring. Refer to Spark Plug Replacement .
  2. Visually inspect the spark plug electrodes for excessive fouling. Refer to Spark Plug Inspection .

Did you find and correct the condition?

--

Go to Symptoms - Engine Mechanical in Engine Mechanical

Go to Step 21

21

  1. Visually inspect spark plug insulators for cracks, carbon tracking, or other damage. Refer to Spark Plug Inspection .
  2. Inspect spark plug electrodes for incorrect gap. Refer to Ignition System Specifications .
  3. If a condition is found, repair as necessary. Refer to Spark Plug Replacement .

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

22

  1. Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical in Engine Mechanical-3.8L
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 23

23
  1. Inspect for a transaxle torque converter clutch (TCC) condition. Refer to Torque Converter Diagnosis in Automatic Transaxle - 4T65-E.
  2. If a condition is found, repair the transaxle as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Diagnostic Aids

24

Replace the ICM Refer to Ignition Control Module Replacement .

Did you complete the replacement?

--

Go to Step 25

--

25
  1. Use a 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.

Does the DTC run and pass?

--

Go to Step 26

Go to Step 2

26

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