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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2006 🢒 Terraza 🢒 Brakes 🢒 Park Brake 🢒 DTC P0300

DTC Descriptor

DTC P0300:  Engine Misfire Detected

Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit/System Description

The engine control module (ECM) uses information from the ignition control module (ICM) and the camshaft position (CMP) sensor 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 the 3-way catalytic converter (TWC) to overheat under certain driving conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets.

Conditions for Running the DTC
    • DTCs P0016, P0101, P0102, P0103, P0107, P0108, P0116, P0117, P0118, P0120, P0125, P0128, P0220, P0315, P0335, P0336, P0340, P0341, P0608, P1516, P2101, P2120, P2125, P2135, or P2138 is not set.
    • The engine speed is between 525-6,000 RPM.
    • The ignition voltage is between 10-18 volts.
    • The ECT parameter is between -7 and +123°C (+19 and +253°F).
    • When start-up ECT is less than -7°C (+19°F), this diagnostic is disabled, until 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.
    • The transmission is not changing gears.
    • The ECM is not in fuel shut-off or decel fuel cut-off mode.
    • The ECM is not receiving a rough road signal.
    • The power management is not active.
    • Excessive drive wheel slip is not detected.

Conditions for Setting the DTC

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

Action Taken When the DTC Sets

The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second ignition cycle the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:

    • The control module detects the same misfire failure during 2 consecutive trips.
    • The control module detects any misfire failure during any subsequent trip if the conditions at the time of failure meet the following criteria:
       - The engine load is within 20 percent of the previous test that failed.
       - The engine speed is within 375 RPM of the previous test that failed.
       - The engine coolant temperature is in the same range of the previous test that failed.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) at the beginning of the fourth ignition cycle, after 3 ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC and related Freeze Frame data clears after 40 warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

You must perform the Crankshaft Position System Variation Learn before proceeding with this diagnostic procedure.

If condition is intermittent or not active, refer to Testing for Intermittent Conditions and Poor Connections .

Reference Information

Schematic Reference

    •  Engine Controls Connector End Views
    •  Ground Distribution Schematics

Electrical Information Reference

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

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Output Controls

Special Tools Required

J 26792 Spark Tester

Circuit/System Verification
  1. There should be no abnormal engine noise.
  2. Diagnose any other DTCs that are set. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  3. Observe the Misfire Current Counters with a scan tool.
  4. Operate the engine within the conditions listed in the Freeze Frame Failure Records.
  5. The engine should not be misfiring.

Circuit/System Testing
  1. If P0201-P0206 are set, diagnose those DTCs first.
  2. Operate the engine within the conditions listed in the Freeze Frame Failure Records.
  3. A current misfire should not be indicated.
    4. ⇒If a single cylinder Current Misfire is indicated, inspect the following components of the affected cylinder for proper operation:
    • The spark plug--Refer to Spark Plug Inspection .
    • The fuel injector--Refer to Fuel Injector Diagnosis .
    ⇒If more than one cylinder is randomly misfiring, inspect the following components condition and operation:
    • The ignition coil--Refer to Electronic Ignition (EI) System Diagnosis .
    • The vacuum hoses and seals--Refer to Evaporative Emissions Hose Routing Diagram .
    • The throttle body and intake manifold
    • The brake booster hose.
    • The crankcase ventilation system--Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • The ECM grounds
    • The exhaust system--Refer to Restricted Exhaust .
    • The fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • The spark plugs--Refer to Spark Plug Inspection .

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Repair Verification

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

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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.
    5. ⇒If the DTC fails this ignition, a misfire still exists.
  5. Operate the vehicle at 2,500 RPM for 4 minutes.
  6. Operate the vehicle within the Conditions for Running DTC P0420. Refer to DTC P0420 .
    7. ⇒If DTC P0420 does not run and pass, diagnose that DTC.