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DTC P0300: Random/Multiple Cylinder Misfire Detected: Notes

DTC detecting condition: 

Trouble symptom: 

CAUTION: After servicing or replacing faulty parts, perform Clear Memory Mode Ref. to ENGINE (DIAGNOSTICS) (STI)>Clear Memory Mode>OPERATION , and inspection mode Ref. to ENGINE (DIAGNOSTICS) (STI)>Inspection Mode>PROCEDURE .

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .

G12456923Courtesy of SUBARU OF AMERICA, INC.
  1. CHECK OUTPUT SIGNAL OF ECM  .
    1. Turn the ignition switch to ON.
    2. Measure the voltage between ECM connector and chassis ground on all cylinders.

      Connector & terminal 

      #1 (B134) No. 10 (+) - Chassis ground (-):

      #2 (B134) No. 11 (+) - Chassis ground (-):

      #3 (B134) No. 12 (+) - Chassis ground (-):

      #4 (B134) No. 13 (+) - Chassis ground (-):

    Is the voltage 10 V or more?

    Yes:  Go to  6

    No:  Go to  2

  2. CHECK HARNESS BETWEEN ECM AND FUEL INJECTOR CONNECTOR  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from all fuel injectors.
    3. Measure the resistance between all fuel injector connectors and engine ground.

      Connector & terminal 

      #1 (E5) No. 2 - Engine ground:

      #2 (E16) No. 2 - Engine ground:

      #3 (E6) No. 2 - Engine ground:

      #4 (E17) No. 2 - Engine ground:

    Is the resistance 1 MΩ or more?

    Yes:  Go to  3

    No:  Repair the short circuit to ground in harness between ECM connector and fuel injector connector.

  3. CHECK HARNESS BETWEEN ECM AND FUEL INJECTOR CONNECTOR  .

    Measure the resistance of harness between ECM connector and fuel injector connector on all cylinders.

    Connector & terminal 

    #1 (B134) No. 10 - (E5) No. 2:

    #2 (B134) No. 11 - (E16) No. 2:

    #3 (B134) No. 12 - (E6) No. 2:

    #4 (B134) No. 13 - (E17) No. 2:

    Is the resistance less than 1 Ω?

    Yes:  Go to  4

    No:  Repair the harness and connector.

    NOTE: In this case, repair the following item:
    • Open circuit in harness between ECM connector and fuel injector connector
    • Poor contact of coupling connector
  4. CHECK FUEL INJECTOR  .

    Check all fuel injectors. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Fuel Injector>INSPECTION .

    Is the check result OK?

    Yes:  Go to  5

    No:  Replace the faulty fuel injector. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Fuel Injector .

  5. CHECK POWER SUPPLY LINE  .
    1. Turn the ignition switch to ON.
    2. Measure the voltage between all fuel injector connectors and the engine ground.

      Connector & terminal 

      #1 (E5) No. 1 (+) - Engine ground (-):

      #2 (E16) No. 1 (+) - Engine ground (-):

      #3 (E6) No. 1 (+) - Engine ground (-):

      #4 (E17) No. 1 (+) - Engine ground (-):

    Is the voltage 10 V or more?

    Yes:  Repair the poor contact of all connectors in fuel injector circuit.

    No:  Repair the harness and connector.

    NOTE: In this case, repair the following item:
    • Open circuit in harness between the main relay connector and fuel injector connector on faulty cylinders
    • Poor contact of coupling connector
    • Poor contact of main relay connector
  6. CHECK HARNESS BETWEEN ECM AND FUEL INJECTOR CONNECTOR  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from all fuel injectors.
    3. Turn the ignition switch to ON.
    4. Measure the voltage between ECM connector and chassis ground on all cylinders.

      Connector & terminal 

      #1 (B134) No. 10 (+) - Chassis ground (-):

      #2 (B134) No. 11 (+) - Chassis ground (-):

      #3 (B134) No. 12 (+) - Chassis ground (-):

      #4 (B134) No. 13 (+) - Chassis ground (-):

    Is the voltage 10 V or more?

    Yes:  Repair the short circuit to power in harness between ECM connector and fuel injector connectors.

    No:  Go to  7

  7. CHECK FUEL INJECTOR  .
    1. Turn the ignition switch to OFF.
    2. Check all fuel injectors. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Fuel Injector>INSPECTION .

    Is the check result OK?

    Yes:  Go to  8

    No:  Replace the faulty fuel injector. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Fuel Injector .

  8. CHECK INSTALLATION OF CAMSHAFT POSITION SENSOR/CRANKSHAFT POSITION SENSOR  .

    Check installation condition of the camshaft position sensor/crankshaft position sensor.

    Is the camshaft position sensor or crankshaft position sensor loosely installed?

    Yes:  Tighten the camshaft position sensor or crankshaft position sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Camshaft Position Sensor . Ref. to FUEL INJECTION (FUEL SYSTEMS) (STI)>Crankshaft Position Sensor .

    No:  Go to  9

  9. CHECK CRANK SPROCKET  .

    Remove the timing belt cover.

    Is the crank sprocket rusted or does it have broken teeth?

    Yes:  Replace the crank sprocket. Ref. to MECHANICAL(STI)>Crank Sprocket .

    No:  Go to  10

  10. CHECK INSTALLATION CONDITION OF TIMING BELT  .

    Turn the crankshaft, and align alignment mark on crank sprocket with alignment mark on cylinder block.

    ST 499987500 CRANKSHAFT SOCKET

    Is the timing belt dislocated from its proper position?

    Yes:  Repair the installation condition of timing belt. Ref. to MECHANICAL(STI)>Timing Belt .

    No:  Go to  11

  11. CHECK FUEL LEVEL  .

    Check remaining amount of fuel.

    Is the fuel meter indication higher than the "Lower" level?

    Yes:  Go to  12

    No:  Refill the fuel so that the fuel meter indication is higher than the "Lower" level, and proceed to the next step. Go to  12

  12. CHECK STATUS OF MALFUNCTION INDICATOR LIGHT  .
    1. Clear the memory using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (STI)>Clear Memory Mode .
    2. Start the engine, and drive the vehicle for 10 minutes or more at 60 km/h.

    Does the malfunction indicator light illuminate or blink?

    Yes:  Go to  13

    No:  Repair the poor contact of connector.

    NOTE: In this case, repair the following item:
    • Poor contact of ignition coil connector
    • Poor contact of fuel injector connector on faulty cylinders
    • Poor contact of ECM connector
    • Poor contact of coupling connector

    Go to  13

  13. CHECK AIR INTAKE SYSTEM  .

    Check the installing condition of the air intake system.

    Is there any fault in air intake system?

    Yes:  Repair the air intake system.

    NOTE: Check the following items.
    • Are there air leaks or air suction caused by loose or dislocated nuts and bolts?
    • Are there cracks or any disconnection of hoses?

    No:  Go to  14

  14. CHECK PCV VALVE  .

    Check the PCV valve. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (STI)>PCV Valve>INSPECTION .

    Is the check result OK?

    Yes:  Repair or replace the PCV valve. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (STI)>PCV Valve .

    No:  Go to  15

  15. CHECK PCV HOSE ASSEMBLY  .

    Check the PCV hose assembly.

    Is the PCV hose assembly deformed, cracked or damaged?

    Yes:  Repair or replace the PCV hose assembly. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (STI)>PCV Hose Assembly .

    No:  Go to  16

  16. CHECK PCV HOSE ASSEMBLY  .

    Check the clamp of PCV hose assembly.

    Is the clamp of PCV hose assembly damaged or loose?

    Yes:  Repair or replace the clamp of PCV hose assembly. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (STI)>PCV Hose Assembly .

    No:  Go to  17

  17. CHECK ALL CYLINDERS  .

    Check the following items for the cylinders.

    • Spark plug
    • Ignition coil
    • Fuel injector
    • Compression
    • Skipping timing chain teeth

    Is there a fault in any cylinder?

    Yes:  Repair or replace the faulty part of the faulty cylinder.

    NOTE: Check the following items.
    • Spark plug
    • Fuel injector
    • Ignition coil
    • Compression
    • Skipping timing belt teeth

    No:  Go to DTC P0171. Ref. to ENGINE (DIAGNOSTICS) (STI)>Diagnostic Procedure with Diagnostic Trouble Code (DTC)>DTC P0171 SYSTEM TOO LEAN BANK 1 .

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