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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 Allure 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P0300-P0306

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0300: Engine Misfire Detected

DTC P0301: Cylinder 1 Misfire Detected

DTC P0302: Cylinder 2 Misfire Detected

DTC P0303: Cylinder 3 Misfire Detected

DTC P0304: Cylinder 4 Misfire Detected

DTC P0305: Cylinder 5 Misfire Detected

DTC P0306: Cylinder 6 Misfire Detected

Circuit/System Description

The engine control module (ECM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensors in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. A misfire rate that is high enough can cause 3-way catalytic converter damage. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for catalytic converter damage are present. DTCs P0301 through P0306 correspond to cylinders 1 through 6. If the ECM is able to determine that a specific cylinder is misfiring, the DTC for that cylinder will set.

Conditions for Running the DTC
    • DTCs P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, P0338, P0461, P0462, P0463, and P0464 are not set.
    • The engine speed is between 400-7,000 RPM and steady.
    • The increase in engine speed is within a range of 2,500-4,600 RPM/Sec, dependant on the rate of throttle movement.
    • The delivered torque signal is greater than 12 percent at idle.
    • The delivered torque signal is between 11-29 percent with the transmission in drive.
    • The intake air temperature (IAT) is greater than -30°C (-22°F).
    • The fuel level is greater than 12 percent.
    • The torque management is not active.
    • The antilock brake system (ABS)/traction control system (TCS) is not active.
    • The ECM is not receiving a rough road signal.
    • The fuel cut-off is not active, including the traction control, the deceleration, the high vehicle speed, and the high engine speed.
    • DTCs P0300 through P0306 run continuously when the above conditions exist for greater than 1000 engine revolutions.
        OR
    • 200 revolutions after the first occurrence for a catalyst damaging misfire. The coolant temperature must be less than 47°C (117°F), and the fuel level greater than 12 percent, or less than 12 percent if the MIL is flashing.

Conditions for Setting the DTC

P0300

The ECM detects a crankshaft rotation speed variation indicating a misfire rate sufficient to cause emissions levels to exceed a predetermined value for greater than 4 seconds or for a cumulative of 50 seconds.

P0301, P0302, P0303, P0304, P0305, P0306

The ECM detects a crankshaft rotation speed variation indicating a single cylinder misfire rate sufficient to cause emissions levels to exceed mandated standards for greater than 4 seconds or for a cumulative of 50 seconds.

Action Taken When the DTC Sets

DTCs P0300, P0301, P0302, P0303, P0304, P0305, and P0306 are Type A (MIL flashing) or Type B1 DTCs.

Conditions for Clearing the DTC

DTCs P0300, P0301, P0302, P0303, P0304, P0305, and P0306 are Type A (MIL flashing) or Type B1 DTCs.

Diagnostic Aids
    • A misfire DTC could be caused by an excessive vibration from sources other than the engine. Inspect for the following possible sources:
       - A tire or wheel that is out of round or out of balance
       - Variable thickness brake rotors
       -  An unbalanced drive shaft
       - Certain rough road conditions
       - Transmission operation
       - A damaged accessory drive component or belt
    • High resistance in the circuits of the injectors could set a misfire DTC without setting an injector DTC. Test the injector circuits of the affected cylinder(s) for a high resistance if you suspect a condition.

Reference Information

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Special Tools

J 26792 HEI Spark Tester

Circuit/System Verification
  1. Engine idling at the normal operating temperature.
    2. ⇒If there is an abnormal engine noise, refer to Symptoms - Engine Mechanical .
  2. Verify that DTCs: DTC P0011, P0014, P0021, P0024, P0201-P0206, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P0335, P0336, P0338, P2088, P2090, P2092, P2094, P2300, P2301, P2303, P2304, P2306, P2307, P2309, P2310, P2312, P2313, P2315, or P2316 are not set.
    3. ⇒If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  3. Observe the Misfire Current Cyl. 1-6 parameters with a scan tool. The Misfire Current Counters should not be incrementing.
  4. Engine idling, perform the cylinder power balance test with a scan tool to isolate the misfiring cylinder. The engine speed should change when each injector is disabled.
  5. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

Circuit/System Testing
  1. Verify that the following conditions do not exist:
    2. • Vacuum hose splits, kinks, and incorrect connections
    • Engine vacuum leaks
    • Crankcase ventilation system for vacuum leaks
    • Fuel pressure that is too low or too high--Refer to Fuel System Diagnosis .
    • Contaminated Fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Exhaust system restrictions
         ⇒If you find any of the above conditions, repair as necessary.
  2. Ignition OFF, remove the ignition coil of the misfiring cylinder, but leave the electrical connector connected.
  3. Inspect the ignition coil boot for the following conditions:
    4. • Holes
    • Tears
    • Carbon tracking
    • Oil contamination
    • Water intrusion
         ⇒If you find any of the above conditions, repair as necessary.
  4. Remove the fuel pump fuse from the fuse block.
  5. Install the J 26792 to the boot of the ignition coil and ground.

    6. Important: An erratic or weak spark is considered a no spark condition.

  6. Attempt to start the engine and observe the J 26792 . The spark tester should spark.
    7. ⇒If there is no spark, refer to Electronic Ignition (EI) System Diagnosis for diagnosis of the ignition coil.
  7. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the spark plug does not exhibit the following conditions:
    8. • Gas, coolant, or oil fouled--Refer to Spark Plug Inspection .
    • Cracked, worn, incorrectly gapped--Refer to Ignition System Specifications .
    ⇒If there is a condition with the spark plug, replace the spark plug.
  8. Exchange the suspected spark plug with another cylinder that is operating correctly.
  9. Engine idling, observe the Misfire Current Counters with a scan tool. The misfire should not follow the spark plug exchange.
    10. ⇒If the misfire follows the spark plug, replace the spark plug.
  10. If all conditions test normal, test or inspect for the following:
    11. • A lean or rich fuel injector--Refer to Fuel Injector Solenoid Coil Test .
    • An engine mechanical condition--Refer to Symptoms - Engine Mechanical .

Repair Instructions
    •  Spark Plug Replacement
    •  Ignition Coil Replacement - Bank 1
    •  Ignition Coil Replacement - Bank 2

Repair Verification
  1. If the customer concern was a flashing malfunction indicator lamp (MIL), refer to DTC P0420 .
  2. Clear the DTCs with a scan tool.
  3. Turn OFF the ignition for 30 seconds.
  4. Start the engine.
  5. 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/Failure Records.
    6. ⇒If the DTC fails this ignition, a misfire still exists.