GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 STS 🢒 Engine 🢒 Engine Controls and Fuel - 2.8L, 3.0L, 3.2L, or 3.6L 🢒 DTC P0300-P0306

DTC P0300-P0306 with LLT

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, and the corresponding fuel injector is disabled. The ECM will re-enable the fuel injector after 4 crankshaft revolutions when the misfire is not current. The ECM will re-enable the fuel injector after 8 crankshaft revolutions when the misfire is not current, if the vehicle is equipped with a manual transmission.

Conditions for Running the DTC
    • DTC P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, or P0338 is not set.
    • The engine speed is between 420-7,000 RPM and steady.
    • The evaporative emissions (EVAP) leak detection is not active.
    • The delivered torque signal is greater than 5 percent at idle.
    • The delivered torque signal is between 6-29 percent with the transmission in drive.
    • The intake air temperature (IAT) is greater than -30°C (-22°F).
    • The A/C compressor clutch is not changing states.
    • The ECM is not receiving a rough road signal.
    • The fuel level is greater than 11 percent.
        OR
    • The fuel level is less than 11 percent and the MIL is illuminated for a misfire DTC, or a fuel level sensor DTC is set.
    • The ECM is not in fuel cut-off or decel fuel cut-off mode.
    • The throttle angle is steady.
    • DTCs P0300 through P0306 run continuously when the above conditions exist for at least 1,000 engine revolutions, 200 revolutions after the first occurrence.

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.

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.

Action Taken When the DTC Sets
    • DTCs P0300, P0301, P0302, P0303, P0304, P0305, and P0306 are Type A (MIL flashing) or Type B1 DTCs.
    • The ECM disables the fuel injector of the misfiring cylinder when a catalyst damaging misfire is present.

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

Connector End View Reference

Component Connector End Views

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 DTC P000A, P000B, P000C, P000D, P0011, P0014, P0021, P0024, P0087, P0088, P0089, P0201-P0206, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P029D, P02A1, P02A5, P02A9, P02AD, P02B1, P0335, P0336, P0338, P0351-P0356, P2088, P2090, P2092, P2094, P2146, P2149, P2152, P2155, P216A, P216D, P2300, P2301, P2303, P2304, P2306, P2307, P2309, P2310, P2312, P2313, P2315, or P2316 is not set.
    3. ⇒If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.
  3. Observe the scan tool Misfire Current Cyl. 1-6 parameters. The Misfire Current Counters should not be incrementing.
  4. Observe the scan tool Circuit Test Status parameters for the various components. Verify the Circuit Test Status parameters for the components that may cause a misfire display OK or Not Run.
  5. 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.
  6. 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 you observed from the Freeze Frame/Failure Records data.

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 appropriate ignition coil and ground.

    6. Note: 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 scan tool Misfire Current Counter parameters. 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.
    • 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 or P0430.
  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 to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
    6. ⇒If the DTC fails this ignition, a misfire still exists.