GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 XLR 🢒 DTC 🢒 DTC P0300-P0308

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

DTC P0307: Cylinder 7 Misfire Detected

DTC P0308: Cylinder 8 Misfire Detected

Circuit/System Description

The engine control module (ECM) uses information from the crankshaft position (CKP) sensor in order to determine when an engine misfire is occurring and uses information from the camshaft position (CMP) sensor in order to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets. Under certain driving conditions, a misfire rate can be high enough to cause the 3-way catalytic converter (TWC) to overheat, possibly damaging the converter. The malfunction indicator lamp (MIL) will flash ON and OFF when converter overheating, damaging conditions are present and DTC P0300 is set. DTCs P0301-P0308 correspond to cylinders 1-8. 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 P0101, P0102, P0103, P0106, P0107, P0108, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0128, P0220, P0222, P0223, P0335, P0336, P2135, or P2176 is not set.
    • DTC P0315 is set and the engine speed is less than 1000 RPM
        OR
         DTC P0315 is not set and the engine speed is between 650-6,000 RPM.
    • The ignition voltage is between 9-18 volts.
    • The engine coolant temperature (ECT) is between -7 and +125°C (+19 and +257°F).
    • If the ECT is less than -7°C (19°F) at engine start-up, the diagnostic will be disabled until the ECT is more than 21°C (69°F).
    • The fuel level is more than 10 percent.
    • The ABS and the traction control system (TCS) are not active or detecting a rough road condition.
    • The ECM is not in fuel shut-off or decel fuel cut-off mode.
    • The power management is not active.
    • Excessive drive wheel slip is not detected.
    • DTCs P0300, and P0301-P0308 run continuously when the above conditions are met.

Conditions for Setting the DTC

The ECM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission or catalytic converter damaging levels to exceed mandated standards.

Action Taken When the DTC Sets
    • DTCs P0300-P0308 are Type B DTCs.
    • When the MIL is flashing the injector may be disabled for the misfiring cylinder to protect the catalytic converter.
    • Depending on the conditions that set the code, the engine may go into open loop.

Conditions for Clearing the MIL/DTC
    • DTCs P0300-P0308 are Type B DTCs.
    • The DTC must pass the same conditions that were present when the DTC failed.

Diagnostic Aids
    • A misfire may only occur when the engine is under a load or when the engine is cold.
    • The Crankshaft Position System Variation Learn may need to be performed.
    • 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
       - A damaged accessory drive component or belt

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 Spark Tester

Circuit/System Verification
  1. Verify that no other DTCs are set.
    2. ⇒If any other DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Engine idling, verify there is no abnormal engine noise.
    3. ⇒If there is abnormal engine noise, refer to Symptoms - Engine Mechanical .
  3. Observe the Misfire Current Counters with a scan tool. The Misfire Current Counters should not be incrementing. Random misfires that are not steadily incriminating may be normal.
  4. Engine running, observe the DTC information with a scan tool. DTCs P0300-P0308 should not set.
  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 data.

Circuit/System Testing

Important: You must perform the Circuit/System Verification before proceeding with the Circuit/System testing.

  1. Verify that the following conditions do not exist:
    2. • Engine 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 .
    • Restricted exhaust system--Refer to Restricted Exhaust .
         ⇒If you find any of the above conditions, repair as necessary.
  2. Ignition OFF, remove the ignition coil from the misfiring cylinder, leaving the electrical connector connected.
  3. Install the J 26792 to the boot of an ignition coil and ground.

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

  4. Attempt to start the engine and observe the J 26792 . The spark tester should spark.
    5. ⇒If there is no spark, refer to Electronic Ignition (EI) System Diagnosis for diagnosis of the ignition coil.
  5. Ignition OFF, remove the spark plug from the misfiring cylinder. Verify that the following conditions do not exist with the spark plug:
    6. • Gas, coolant, or oil fouled
    • Cracked, worn, incorrectly gapped--Refer to Spark Plug Inspection .
         ⇒If there is a condition with the spark plug, replace the spark plug.
  6. Exchange the suspected spark plug with another cylinder that is operating correctly.
  7. Operate the engine under the conditions in which the misfire occurred. This may include putting the engine under a load, turning on the A/C, seat heaters, or rear window defogger. Observe the Misfire Current Counters with a scan tool. The misfire should not follow the spark plug exchange.
    8. ⇒If the misfire follows the spark plug, replace the spark plug.
  8. If all conditions test normal, test or inspect for the following:
    9. • A lean or rich fuel injector--Refer to Fuel Injector Diagnosis .
    • An engine mechanical condition--Refer to Symptoms - Engine Mechanical .

Repair Verification

If the customer concern was a flashing MIL, perform the following procedure:

  1. Install any components or connectors that have been removed or replaced during diagnosis.
  2. Perform any adjustment, programming or setup procedures that are required when a component or module is removed or replaced.
  3. Clear the DTCs.
  4. Turn OFF the ignition for 60 seconds.
  5. If the repair was related to a DTC, duplicate the Conditions for Running the DTC and use the Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to the Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
  6. To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420 or P0430. Refer to DTC P0420 or P0430 .