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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 Regal 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 DTC P0300 Engine Misfire Detected

Circuit Description

The powertrain control module (PCM) has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the ignition control module. The PCM monitors crankshaft speed variations, reference period differences, to determine if a misfire is occurring. If 2 percent or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 22 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible three-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the malfunction indicator lamp (MIL) will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Running the DTC
    • No VSS, TP Sensor, MAP Sensor, ECT Sensor, CKP Sensor CMP Sensor, or MAF Sensor DTCs set.
    • Engine speed is between 550-5850 RPM.
    • The system voltage is between 9-18 volts.
    • The ECT indicates an engine temperature between -6°C-120°C (21°F-248°F).
    • Throttle angle is steady.

Conditions for Setting the DTC

The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels to exceed mandated standard.

Action Taken When the DTC Sets
    • The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed unless three-way converter damage is possible in which case the lamp will illuminate after the first failure.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

Conditions for Clearing the MIL/DTC
    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using a scan tool.

Diagnostic Aids

The scan tool provides information that can be useful in identifying the misfiring cylinder. If 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 sensor have recently been replaced, and the CKP System Variation Learn Procedure has not been performed. If the diagnostic table does not identify a problem then perform the Crankshaft Position System Variation Learn . The Crankshaft Variation Learn Procedure should be performed if any of the following conditions are true:

The scan tool displayed misfire counter values -- Misfire Hist. #1-#6, can be useful in determining whether the misfire affects a single cylinder, a cylinder pair, cylinders that share an ignition coil, 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 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 crankshaft position sensor has been replaced.
    • Secondary ignition wires -- Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. The wires should measure under 600 ohms per/ft.
    • Damaged or malfunctioning ignition coil -- Check for cracks, carbon tracking, or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000 ohms and 8000 ohms (5K ohms and 8K ohms).
    • Substitute a known good coil -- Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

If the misfire is random, check for the following conditions

    • System grounds -- Ensure all connections are clean and properly tightened.
    • Mass air flow sensor -- A mass air flow (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 fail records conditions with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor, refer to Mass Airflow Sensor Replacement .
    • 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.
    • 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, incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.
    • 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 Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) .
    • Fuel injectors -- Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
    • EGR system -- Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
    • 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 .
    • Extended idle -- Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. 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 DTC P0300 to set, review the Freeze Frame and Failure Records data for DTC P0300. If DTC P0300 occurs at high engine speeds, the condition described above did not cause DTC to set. If 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 to 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.

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.

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: :  Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

    • Loose terminal connection
       -  Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
       -  Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
    • Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.
    •  Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

Test Description

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

  1. A malfunctioning injector circuit, crankshaft position system variation not learned condition, or incorrect rough road data from the 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, 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 through 12 test for conditions that can cause a random cylinder misfire.

    5.

  5. Steps 13 through 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 Powertrain On Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Are any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List/Type

Go to Step 3

3

  1. Start and idle the engine.
  2. Review and record 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.

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

--

Go to Step 4

Go to Diagnostic Aids

4

With a scan tool, view the Misfire History Cyl  # display.

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

--

Go to Step 5

Go to Step 13

5

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

Did you find and correct the condition?

--

Go to Step 25

Go to Step 6

6

  1. Visually and physically inspect the PCV valve for improper installation and for damaged O-rings. Refer to Crankcase Ventilation System Inspection .
  2. If a problem is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 7

7

  1. 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.
  2. If a problem 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

  1. Test fuel pressure. Refer to Fuel System Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) .
  2. If a problem is found, repair as necessary.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 9

9

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

Did you find and correct the condition?

--

Go to Step 25

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 problem 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. Visually and physically inspect the following areas for vacuum leaks:
    2. • Intake manifold
    • EGR adapter
    • EGR valve
    • EGR feed pipes
    • Injector O-rings
  2. If a problem 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 problem 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 - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
  2. If a problem 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 to ensure that they are not damaged and are connected to the proper cylinders at the coils and at the spark plugs.
  2. If a problem is found, repair as necessary. Refer to Spark Plug Wire Harness Replacement in Engine Electrical.

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 Misfire History Cyl # counters as having the most severe misfire, 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.

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

Is a spark present?

--

Go to Step 20

Go to Step 16

16

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

    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

  1. Measure the resistance of the ignition wires associated with the cylinders that were indicated as misfiring.
  2. Replace any ignition wires that measure more than the specified value. Refer to Spark Plug Wire Harness Replacement in Engine Electrical.

Did you find and correct the condition?

7K ohms (7,000 ohms)

Go to Step 25

Go to Step 18

18

  1. Remove and visually and physically inspect the ignition coils associated with the cylinders that were indicated as misfiring. Ensure that the coils and coil towers are free of cracks and carbon tracking.
  2. If a problem 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?

5K-8K ohms

(5000-8000 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 Wire Harness Replacement in Engine Electrical.
  2. Visually inspect the spark plug electrodes for excessive fouling.

Did you find and correct the condition?

--

Go to Base Engine Misfire Diagnosis in Engine Mechanical

Go to Step 21

21

  1. Visually inspect spark plug insulators for cracks, carbon tracking, or other damage.
  2. Inspect spark plug electrodes for incorrect gap.
  3. If a problem is found, repair as necessary. Refer to Spark Plug Wire Harness Replacement in Engine Electrical.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

22

  1. Inspect for an engine mechanical problem:
    2. • Damaged accessory drive belt or pulley.
    • Damaged driven accessory, such as the generator, water pump, drive belt tensioner, etc.
    • Base engine mechanical problem. Refer to Base Engine Misfire Diagnosis in Engine Mechanical.
    • Loose or broken motor mount.
  2. If a problem 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 TCC problem.
  2. If a problem is found, repair the transaxle as necessary. Refer to Torque Converter Diagnosis in Automatic Transaxle.

Did you find and correct the condition?

--

Go to Step 25

Go to Diagnostic Aids

24

Replace the ignition control module. Refer to Ignition Control Module Replacement .

Is the action complete?

--

Go to Step 25

--

25
  1. Use a scan tool to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK