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Home >> Subaru >> 2019 >> WRX STI >> Repair and Diagnosis (Single Page) >> Engine Performance >> Testing & Diagnosis >> Engine Control System (Diagnostics) (STI) (3 Of 4) >> Diagnostic Procedure With Diagnostic Trouble Code (DTC) >> DTC P2096: Post Catalyst Fuel Trim System Too Lean Bank 1 >> Notes

DTC P2096: Post Catalyst Fuel Trim System Too Lean Bank 1: Notes

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

CAUTION: After servicing or replacing faulty parts, perform Clear Memory Mode Refer to ENGINE (DIAGNOSTICS) (STI)>CLEAR MEMORY MODE>OPERATION ., and inspection mode Refer to ENGINE (DIAGNOSTICS) (STI)>INSPECTION MODE>PROCEDURE .

Wiring diagram: 

Engine Electrical System Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

G12456917Courtesy of SUBARU OF AMERICA, INC.
G12456922Courtesy of SUBARU OF AMERICA, INC.
  1. CHECK DTC. 

    Read the engine DTC using the Subaru Select Monitor or general scan tool. Refer to ENGINE (DIAGNOSTICS) (STI)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

    Is other than DTC P2096 displayed? (Current malfunction)

    Yes:  Check DTC using "List of Diagnostic Trouble Code (DTC)". Refer to ENGINE (DIAGNOSTICS) (STI)>LIST OF DIAGNOSTIC TROUBLE CODE (DTC) .

    No:  Go to  2.

  2. CHECK FRONT OXYGEN (A/F) SENSOR CONNECTOR AND COUPLING CONNECTOR. 

    Check the following items of front oxygen (A/F) sensor.

    • Front oxygen (A/F) sensor connector
    • Coupling connector

    Has water entered the connector?

    Yes:  Completely remove any water inside.

    No:  Go to  3.

  3. CHECK HARNESS BETWEEN ECM AND FRONT OXYGEN (A/F) SENSOR CONNECTOR. 
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from ECM.
    3. Disconnect the connectors from front oxygen (A/F) sensor.
    4. Measure the resistance of harness between ECM connector and front oxygen (A/F) sensor connector.

      Connector & terminal 

      (B136) No. 18 - (B379) No. 3:

      (B136) No. 19 - (B379) No. 4:

    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 front oxygen (A/F) sensor connector
    • Poor contact of front oxygen (A/F) sensor connector
    • Poor contact of ECM connector
    • Poor contact of coupling connector
  4. CHECK HARNESS BETWEEN ECM AND FRONT OXYGEN (A/F) SENSOR CONNECTOR. 

    Measure the resistance between ECM connector and chassis ground.

    Connector & terminal 

    (B136) No. 18 - Chassis ground:

    (B136) No. 19 - Chassis ground:

    Is the resistance 1 MΩ or more?

    Yes:  Go to  5.

    No:  Repair the short circuit to ground in harness between ECM connector and front oxygen (A/F) sensor connector.

  5. CHECK OUTPUT SIGNAL FOR ECM. 
    1. Connect the connector to ECM.
    2. Turn the ignition switch to ON.
    3. Measure the voltage between front oxygen (A/F) sensor connector and chassis ground.

      Connector & terminal 

      (B379) No. 4 (+) - Chassis ground (-):

    Is the voltage 4.5 V or more?

    Yes:  Go to  7.

    No:  Go to  6.

  6. CHECK OUTPUT SIGNAL FOR ECM. 

    Measure the voltage between front oxygen (A/F) sensor connector and chassis ground.

    Connector & terminal 

    (B379) No. 3 (+) - Chassis ground (-):

    Is the voltage 4.95 V or more?

    Yes:  Go to  7.

    No:  Go to  8.

  7. CHECK OUTPUT SIGNAL FOR ECM. 

    Measure the voltage between front oxygen (A/F) sensor connector and chassis ground.

    Connector & terminal 

    (B379) No. 3 (+) - Chassis ground (-):

    (B379) No. 4 (+) - Chassis ground (-):

    Is the voltage 8 V or more?

    Yes:  Repair the short circuit to power in the harness between ECM connector and front oxygen (A/F) sensor connector. After repair, replace the ECM. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>ENGINE CONTROL MODULE (ECM) .

    No:  Repair the poor contact of ECM connector.

  8. CHECK EXHAUST SYSTEM. 

    Check the exhaust system.

    Are there holes or loose bolts on exhaust system?

    Yes:  Repair the exhaust system.

    No:  Go to  9.

  9. CHECK AIR INTAKE SYSTEM. 

    Check the air intake system.

    Are there holes, loose bolts or disconnection of hose on air intake system?

    Yes:  Repair the air intake system.

    No:  Go to  10.

  10. CHECK FUEL PRESSURE. 
    WARNING: Place "NO OPEN FLAMES" signs near the working area.
    CAUTION:
    • Be careful not to spill fuel.
    • Before attaching/detaching a fuel pressure gauge, release the fuel pressure.
    1. Connect the front oxygen (A/F) sensor connector.
    2. Measure the fuel pressure while disconnecting pressure regulator vacuum hose from intake manifold. Refer to MECHANICAL (STI)>FUEL PRESSURE>INSPECTION .
    NOTE: If fuel pressure does not increase, squeeze the fuel return hose 2 to 3 times, then measure fuel pressure again.

    Is the measured value 284 - 314 kPa (2.9 - 3.2 kgf/cm2 , 41 - 46 psi)?

    Yes:  Go to  11.

    No:  Check the fuel pump and fuel delivery line. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>FUEL PUMP>INSPECTION . Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>FUEL DELIVERY, RETURN AND EVAPORATION LINES>INSPECTION .

  11. CHECK FUEL PRESSURE. 

    After connecting the pressure regulator vacuum hose, measure fuel pressure. Refer to MECHANICAL (STI)>FUEL PRESSURE>INSPECTION .

    NOTE:
    • If fuel pressure does not increase, squeeze fuel return hose 2 to 3 times, then measure fuel pressure again.
    • If the measured value at this step is out of specification, check or replace pressure regulator and pressure regulator vacuum hose.

    Is the measured value 230 - 260 kPa (2.4 - 2.7 kgf/cm2 , 33 - 38 psi)?

    Yes:  Go to  12.

    No:  Check the fuel pump and fuel delivery line. Refer to FUEL INJECTION (FUEL SYSTEMS (STI)>FUEL PUMP>INSPECTION . Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>FUEL DELIVERY, RETURN AND EVAPORATION LINES>INSPECTION .

  12. CHECK ENGINE COOLANT TEMPERATURE SENSOR. 
    1. Start the engine and warm up completely.
    2. Read the value of «Coolant Temp.» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Is the value of «Coolant Temp.» 75°C (167°F) or more?

    Yes:  Go to  13.

    No:  Replace the engine coolant temperature sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>ENGINE COOLANT TEMPERATURE SENSOR .

  13. CHECK MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR. 
    1. Start the engine and warm up engine until coolant temperature is higher than 75°C (167°F).
    2. Place the gear shift lever in the neutral position.
    3. Turn the A/C switch to OFF.
    4. Turn all the accessory switches to OFF.
    5. Read the value of «Mass Air Flow» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Is the value of «Mass Air Flow» 2 - 5 g/s (0.26 - 0.66 lb/m)?

    Yes:  Go to  14.

    No:  Replace the mass air flow and intake air temperature sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR .

  14. CHECK MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR. 
    1. Start the engine and warm up engine until coolant temperature is higher than 75°C (167°F).
    2. Place the gear shift lever in the neutral position.
    3. Turn the A/C switch to OFF.
    4. Turn all the accessory switches to OFF.
    5. Open the front hood.
    6. Measure the ambient temperature.
    7. Read the value of «Intake Air Temperature» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Subtract ambient temperature from «Intake Air Temperature». Is the obtained value -10 - 50°C (-18 - 90°F)?

    Yes:  Go to  15.

    No:  Check the mass air flow and intake air temperature sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR .

  15. CHECK REAR OXYGEN SENSOR DATA. 
    1. Warm up the engine until engine coolant temperature is higher than 75°C (167°F), and keep the engine speed at 3, 000 rpm. (2 minutes maximum)
    2. Read the value of «Rear O2 Sensor Voltage» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Depress the clutch pedal.
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Is the value of «Rear O2 Sensor Voltage» 0.490 V or more?

    Yes:  Go to  16.

    No:  Go to  17.

  16. CHECK REAR OXYGEN SENSOR DATA. 
    1. Warm up the engine until engine coolant temperature is higher than 75°C (167°F), and rapidly reduce the engine speed from 3, 000 rpm.
    2. Read the value of «Rear O2 Sensor Voltage» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Depress the clutch pedal.
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Is the value of «Rear O2 Sensor Voltage» 0.250 V or less?

    Yes:  Go to  18.

    No:  Go to  17.

  17. CHECK REAR OXYGEN SENSOR CONNECTOR AND COUPLING CONNECTOR. 

    Check the following items of rear oxygen sensor.

    • Rear oxygen sensor connector
    • Coupling connector

    Has water entered the connector?

    Yes:  Completely remove any water inside.

    No:  Go to  19.

  18. CHECK FRONT OXYGEN (A/F) SENSOR USING REAR OXYGEN SENSOR SIGNAL. 
    1. Warm up the engine until engine coolant temperature is higher than 75°C (167°F), then keep the engine idling for 5 minutes or more.
    2. Read the value of «Rear O2 Sensor Volt age» using the Subaru Select Monitor or a general scan tool.
      NOTE:
      • Subaru Select Monitor

        For detailed operation procedures, refer to "Read Current Data For Engine". Refer to ENGINE (DIAGNOSTICS) (STI)>SUBARU SELECT MONITOR .

      • General scan tool

        For detailed operation procedures, refer to the general scan tool operation instructions.

    Is the value in «Rear O2 Sensor Voltage» kept at 0.250 V or less for 5 minutes or more?

    Yes:  Replace the front oxygen (A/F) sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>FRONT OXYGEN (A/F) SENSOR .

    No:  Go to  19.

  19. CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR 

    CONNECTOR. 

    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from ECM.
    3. Disconnect the connector from rear oxygen sensor.
    4. Measure the resistance of harness between ECM connector and rear oxygen sensor connector.

      Connector & terminal 

      (B136) No. 20 - (T6) No. 3:

      (B135) No. 30 - (T6) No. 4:

    Is the resistance less than 1 Ω?

    Yes:  Go to  20.

    No:  Repair the harness and connector.

    NOTE: In this case, repair the following item:
    • Open circuit in harness between ECM connector and rear oxygen sensor connector
    • Poor contact of coupling connector
  20. CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR. 
    1. Connect the connector to ECM.
    2. Turn the ignition switch to ON.
    3. Measure the volt age between rear oxygen sensor connector and chassis ground.

      Connector & terminal 

      (T6) No. 3 (+) - Chassis ground (-):

    Is the voltage 0.2 - 0.5 V?

    Yes:  Replace the rear oxygen sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (STI)>REAR OXYGEN SENSOR .

    No:  Repair the harness and connector.

    NOTE: In this case, repair the following item:
    • Open circuit in harness between ECM connector and rear oxygen sensor connector
    • Poor contact of ECM connector
    • Poor contact of coupling connector
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