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DTC P0300: Multiple Cylinder Misfire: Testing

  1. Turn ignition on. Using scan tool, read DTCs. If MISFIRE GOOD TRIP counter is displayed and displayed count is "0", go to next step. If MISFIRE GOOD TRIP counter is not displayed or displayed count is not "0", go to step  4 .
  2. At this time, conditions required to set DTC are present. Using scan tool, read and record FREEZE FRAME and SIMILAR CONDITIONS data. Operate vehicle using data in SIMILAR CONDITIONS. While in SIMILAR CONDITIONS screen, go to WHICH CYLINDER IS MISFIRING monitor. If scan tool is counting misfires at this time, go to next step. If scan tool is not counting misfires at this time, no problem is indicated. Test is complete.
  3. Using scan tool, read and record FREEZE FRAME data. Using freeze frame data, attempt to determine cause of misfire. If adaptive fuel percentages are not more than plus or minus 15 percent, go to next step. If adaptive fuel percentages are more than plus or minus 15 percent, check fuel delivery. See FUEL SYSTEMS under SYSTEM & COMPONENT TESTING article.
  4. At this time, engine misfire does not exist or is an intermittent problem. Using scan tool, read and record FREEZE FRAME and SIMILAR CONDITIONS data. Road test vehicle under conditions in FREEZE FRAME and SIMILAR CONDITIONS data, paying particular attention to fault setting conditions such as speed, temperature, load and MAP vacuum. Using scan tool, read DTCs. If DTC returns, go to next step. If DTC does not return, no problem is indicated at this time. Test is complete.
  5. Using scan tool, read and record FREEZE FRAME data. Using freeze frame data, attempt to determine cause of misfire. If adaptive fuel percentages are more than plus or minus 15 percent, check fuel delivery. See FUEL SYSTEMS under SYSTEM & COMPONENT TESTING article. If adaptive fuel percentages are not more than plus or minus 15 percent, go to next step.
  6. Using scan tool, read and record FREEZE FRAME and ENGINE RPM data. Using freeze frame data, attempt to determine cause of misfire. If engine speed is less than 3000 RPM and/or engine coolant temperature is not within normal range, go to next step. If engine speed is more than 3000 RPM and engine coolant temperature is within normal operating range, use lab scope to test CMP and CKP sensors, and check valve timing and engine vacuum.
  7. Check the following additional items as possible mechanical problems:
    • Engine vacuum must be at least 13 in. Hg with shift selector in Neutral.
    • Engine valve timing must be within specification.
    • Engine compression must be within specification.
    • Engine exhaust system must be free of restrictions.
    • Engine Closed Crankcase Ventilation (CCV) system must flow freely.
    • Torque converter stall speed must be within specification.
    • Power brake booster must have no internal leaks.
    • Fuel must be free of contamination.
    • Fuel injectors must be free of restrictions.
    • Fuel injector control wires must be installed on correct injector.

    Repair mechanical problems as necessary. If no mechanical problems exist, go to next step.

  8. Using scan tool, read and record FREEZE FRAME and LOAD VALUE data. Using freeze frame data, attempt to determine cause of misfire. If load value is 50 percent or less and/or engine coolant temperature is not within normal operating range, go to next step. If load value is more than 50 percent and engine coolant temperature is within normal operating range, check secondary ignition, compression and cylinder leakage.
  9. Check following items for possible causes of cylinder misfire.
    • Ignition Coil Circuit
    • Spark Plugs
    • Engine Mechanical Problem
    • PCM Power Or Grounds
    • Irregular CMP Sensor Signal
    • Irregular CKP Sensor Signal
    • Intake Restriction
    • Damaged Trigger Wheel
    • Accessory Drive Belt

    If any problems are found, repair as necessary. If no problems are found, test is complete. Check for any related technical service bulletins that may apply.

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