DTC P0300 Random/Multiple Cylinder Misfire Detected: Notes

DTC detecting condition: 

• Detected when two consecutive driving cycles with fault occur.
• Immediately at fault recognition (A misfire which could damage catalyst occurs.)

Trouble symptom: 

• Engine stalls.
• Improper idling
• Rough driving

Wiring diagram: 

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

Fig 1: Random/Multiple Cylinder Misfire Detected - Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK OUTPUT SIGNAL OF ECM  .
   1. Turn the ignition switch to ON.
   2. Measure the voltage between ECM and chassis ground on all cylinders.

   Connector & terminal 

   #1 (B135) No. 23 (+) - Chassis ground (-):

   #2 (B135) No. 8 (+) - Chassis ground (-):

   #3 (B135) No. 9 (+) - Chassis ground (-):

   #4 (B135) No. 22 (+) - 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. 1 - Engine ground:

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

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

   #4 (E17) No. 1 - 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 and fuel injector connector on all cylinders.

   Connector & terminal 

   #1 (B135) No. 23 - (E5) No. 1:

   #2 (B135) No. 8 - (E16) No. 1:

   #3 (B135) No. 9 - (E6) No. 1:

   #4 (B135) No. 22 - (E17) No. 1:

   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  .

   Measure the resistance between all fuel injector terminals.

   Terminals 

   No. 1 -No. 2:

   Is the resistance 5 - 20 Ω?

   • Yes: Go to 5  .
   • No: Replace the faulty fuel injector. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>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. 2 (+) - Engine ground (-):

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

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

   #4 (E17) No. 2 (+) - 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 main relay connector and fuel injector connector
     • 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 and chassis ground on all cylinders.

   Connector & terminal 

   #1 (B135) No. 23 (+) - Chassis ground (-):

   #2 (B135) No. 8 (+) - Chassis ground (-):

   #3 (B135) No. 9 (+) - Chassis ground (-):

   #4 (B135) No. 22 (+) - 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. Measure the resistance between all fuel injector terminals.

   Terminals 

   No. 1 -No. 2:

   Is the resistance 5 - 20 Ω?

   • Yes: Go to 8  .
   • No: Replace the faulty fuel injector. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>Fuel Injector .
8. CHECK INSTALLATION CONDITION OF 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) (H4DO)>Camshaft Position Sensor>INSTALLATION . Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>Crankshaft Position Sensor>INSTALLATION .
   • No: Go to 9  .
9. CHECK CRANKSHAFT POSITION SENSOR PLATE  .

   Is the crankshaft position sensor plate rusted or does it have broken teeth?

   • Yes: Replace the crankshaft position sensor plate. Ref. to MECHANICAL(H4DO)>Cylinder Block .
   • No: Go to 10  .
10. CHECK INSTALLATION CONDITION OF TIMING CHAIN  .

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

    ST 18252AA000 CRANKSHAFT SOCKET 

    Is the timing chain dislocated from its proper position?

    • Yes: Correct the installation condition of timing chain. Ref. to MECHANICAL(H4DO)>Timing Chain Assembly .
    • No: Go to 11  .
11. CHECK FUEL LEVEL  .

    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  14.
12. CHECK PCV VALVE  .

    Check the PCV valve.

    Is the PCV valve free from deformation, crack or other damage?

    • Yes: Repair or replace the PCV valve. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>PCV Valve .
    • No: Go to 13  .
13. CHECK PCV HOSE  .

    Check the PCV hose.

    Is the PCV hose free from deformation, crack or other damage?

    • Yes: Repair or replace the PCV hose. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>PCV Hose .
    • No: Go to 14  .
14. CHECK STATUS OF MALFUNCTION INDICATOR LIGHT  .
    1. Clear the memory using the Subaru Select Monitor. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Clear Memory Mode .
    2. Start the engine, and drive the vehicle 10 minutes or more.

    Does the malfunction indicator light illuminate or blink?

    • Yes: Go to  16.
    • No: Go to 15  .
15. CHECK CAUSE OF MISFIRE  .

    Was the cause of misfire identified when the engine is running?

    • Yes: Finish diagnostics operation, if the engine has no abnormality.
    • 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
16. CHECK 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 17  .
17. CHECK ALL CYLINDERS  .

    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
      • Ignition coil
      • Fuel injector
      • Compression
      • Skipping timing chain teeth
    • No: Go to DTC P0171. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Diagnostic Procedure with Diagnostic Trouble Code (DTC)>DTC P0171 SYSTEM TOO LEAN BANK 1 .