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For 1990-2009 cars only
Makes 🢒 Buick 🢒 1999 🢒 Regal 🢒 DTC 🢒 DTC P0300 Engine Misfire Detected

Circuit Description

The PCM has the ability to detect a misfire by monitoring the 3X reference from the Ignition Control (IC) Module and camshaft position input signals from the Camshaft Position (CMP) Sensor. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring.

If 2% 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 MIL will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Running the DTC
    •  No VSS, TP, MAP, ECT, CKP, CMP, or MAF sensors DTCs set.
    •  Engine speed between 550 and 5900 RPM.
    •  System voltage between 9.0 and 18.0 volts.
    •  The ECT indicates an engine temperature between -6°C (21°F) and 120°C (248°F).
    •  Throttle angle 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 malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
    • 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 MIL during the third consecutive trip in which the diagnostic has been 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 the 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 Test failed since code clear, the misfire history counters (Misfire Hist #1- #6) will still contain a value that represents the level of misfire for each cylinder.

A misfire DTC may set if components that affect the Crankshaft Position Sensor have recently been replaced, and the Crankshaft Position System Variation Learn has not been performed. If the diagnostic table does not identify a problem then perform the Crankshaft Position System Variation Learn . The Crankshaft Position Variation Learn Procedure should be performed if any of the following conditions are true:

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.

    •  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.

The scan tool displayed misfire counter values (Misfire Hist. #1 through #6) can be useful in determining whether the misfire is isolated to a single cylinder or to a cylinder pair (cylinders that share an ignition coil-1/4, 2/5, 3/6.) If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:

    •  Secondary Ignition Wires: Check wires for affected cylinder pair for disconnected ignition wires or for excessive resistance 600ohms per foot (1 968ohms per meter).
    •  Damaged Or Faulty Ignition Coil: Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be (the wires should measure under 8Kohms (8000ohms).
    •  Substitute a Known Good Coil: Swap 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.
    •  MAF: A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.
    •  Air Induction System: Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty 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 faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.
    •  Injector(s): Perform injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O rings.
    •  EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description

Number(s) below refer to the step number(s) 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 cylinder(s); values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinder(s). When investigating a misfire, always start with items associated with the cylinder(s) that has the largest number of counts stored in the Misfire History Cyl # counter.

    4.

  4. Steps 5 through 12 check for conditions that can cause a random cylinder misfire.

    5.

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

DTC P0300 - Engine Misfire Detected

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Are any other DTCs also set?

--

Go to Applicable DTC

Go to Step 3

3

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

Is Misfire Current # display incrementing for any cylinder (indicating a misfire currently occurring)?

--

Go to Step 4

Go to Diagnostic Aids

4

View the Misfire History Cyl # display on the 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

  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.

Was problem found?

--

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.

Was problem found?

--

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.

    3. Refer to Throttle Body Assembly Replacement .

Was problem found?

--

Go to Step 25

Go to Step 8

8

  1. Check 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.

Was problem found?

--

Go to Step 25

Go to Step 9

9

  1. Check 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.

Was a problem found?

--

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 .
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

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.

Was a problem found?

--

Go to Step 25

Go to Step 12

12

  1. Remove the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .
  2. Visually/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.

Was a problem found?

--

Go to Step 25

Go to Step 13

13

  1. Check for proper fuel injector operation. Refer to the 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.

Was a problem found?

--

Go to Step 25

Go to Step 14

14

  1. Visually and physically inspect the ignition wires associated with the cylinder(s) 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.

Was a problem found?

--

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 (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 spark tester. A spark should be observed.

Is adequate spark present?

--

Go to Step 20

Go to Step 16

16

  1. Remove and visually/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, replace affected ignition wire(s) 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 .

Was a problem found?

--

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 wire(s) that measure greater than the specified value. Refer to Spark Plug Wire Harness Replacement in Engine Electrical.

Was a problem found?

600 ohms Per/Ft

Go to Step 25

Go to Step 18

18

  1. Remove and visually/physically inspect the ignition coil(s) associated with the cylinders that were indicated as misfiring. Ensure that the coil(s) and coil towers are free of cracks and carbon tracking.
  2. If a problem is found, replace damaged ignition coil(s) as necessary. Refer to Ignition Coil Replacement .

Was a problem found?

--

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 coil(s) as necessary. Refer to Ignition Coil Replacement .

Was a problem found?

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 Replacement in Engine Electrical.
  2. Visually inspect the spark plug electrodes for excessive fouling. Refer to Spark Plug Replacement in Engine Electrical.

Was a problem found?

--

Go to Base Engine Misfire Diagnosis

Go to Step 21

21

  1. Visually inspect spark plug insulators for cracks, carbon tracking, or other damage.
  2. Check spark plug electrodes for incorrect gap.
  3. If a problem is found, replace affected spark plug(s) as necessary. Refer to Spark Plug Replacement in Engine Electrical.

Was problem found?

--

Go to Step 25

Go to Step 22

22

  1. Check for an engine mechanical problem.
    2. • Damaged accessory drive belt or pulley.
    • Damaged driven accessory (generator, water pump, drive belt tensioner, ect).
    • Base engine mechanical problem. Refer to Base Engine Misfire Diagnosis .
    • Loose or broken motor mount.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 25

Go to Step 23

23
  1. Check for a transaxle TCC problem. Refer to Functional Test in Automatic Transaxle--4T60-E or Functional Test in Automatic Transaxle--4T65-E.
  2. If a problem is found, repair the transaxle as necessary.

Was a problem found?

--

Go to Step 25

Go to Diagnostic Aids

24

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

Is action complete?

--

Go to Step 25

--

25
  1. Review and record Freeze Frame data
  2. Clear DTCs.
  3. Start and idle the engine.
  4. Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame data).
  5. Monitor the scan tool Misfire Current Cyl # display for each cylinder.

Is Misfire Current Cyl # display incrementing for any cylinder (indicating a misfire currently occurring)?

--

Go to Step 2

System OK