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DTC U0077: Lin Communication Bus "ECM/Pcm" Off: Notes

DTC detecting condition: 

Immediately at fault recognition

CAUTION: Use the check board when measuring the ECM terminal voltage and resistance. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>General Description>PREPARATION TOOL>HOW TO USE CHECK BOARD .
NOTE: After the faulty parts are repaired or replaced, perform the final check in Basic Diagnostic Procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Basic Diagnostic Procedure>PROCEDURE .

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM (CROSSTREK MODEL)>Engine Electrical System>WIRING DIAGRAM .

G12400217Courtesy of SUBARU OF AMERICA, INC.
  1. CHECK DTC  .

    Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) .

    DTC Check 

    NOTE:
    • SUBARU SELECT MONITOR

      Refer to "Diagnostic Trouble Code (DTC)" for detailed operation procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) .

    • General scan tool

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

    Is DTC U0077 displayed? (Current malfunction)

    Yes:  Go to  2.

    No:  Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again. Clear the memory using the Subaru Select Monitor or general scan tool.

    NOTE:
    • SUBARU SELECT MONITOR

      For detailed procedures, refer to "Clear memory". Ref. to COMMON (diag)>CLEAR MEMORY .

    • General scan tool

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

  2. CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (OPEN)  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from ECM.
    3. Disconnect the connector from battery sensor.
    4. Disconnect the connector from the generator.
    5. Disconnect the connector from the active grille shutter.
    6. Measure the resistance of harness between ECM connector and battery sensor connector, generator connector and active grille shutter connector.

      Connector & terminal 

      (B134) No. 5 - (B9) No. 3:

      (B134) No. 5 - (F26) No. 1:

      (B134) No. 5 - (B63) No. 3:

    Is the resistance less than 1 Ω?

    Yes:  Go to  3.

    No:  Repair the harness and connector.

    NOTE: In this case, repair the following item:
    • Open circuit in harness between ECM connector and battery sensor connector
    • Open circuit of harness between ECM connector and generator connector
    • Open circuit of harness between ECM connector and active grille shutter connector
    • Poor contact of coupling connector
  3. CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (SHORT TO GROUND)  .

    Measure the resistance between ECM connector and chassis ground.

    Connector & terminal 

    (B134) No. 5 - Chassis ground:

    Is the resistance 1 MΩ or more?

    Yes:  Go to  4.

    No:  Repair the ground short circuit of harness.

    NOTE: In this case, repair the following item:
    • Harness between ECM connector and battery sensor connector
    • Harness between ECM connector and generator connector
    • Harness between ECM connector and active grille shutter connector
  4. CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (SHORT TO POWER SUPPLY)  .
    1. Turn the ignition switch to ON.
    2. Measure the voltage between ECM connector and chassis ground.

      Connector & terminal 

      (B134) No. 5 - Chassis ground:

    Is the voltage less than 1 V?

    Yes:  Go to  5.

    No:  Repair the short circuit to power supply in harness.

    NOTE: In this case, repair the following item:
    • Harness between ECM connector and battery sensor connector
    • Harness between ECM connector and generator connector
    • Harness between ECM connector and active grille shutter connector
  5. CHECK DTC  .
    1. Turn the ignition switch to OFF.
    2. Connect the connector to the active grille shutter.
    3. Connect the connector to the generator.
    4. Connect the connector to the battery sensor.
    5. Connect the connector to ECM.
    6. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (diag)>CLEAR MEMORY .

      Clear Memory 

    7. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) .

      DTC Check 

      NOTE:
      • SUBARU SELECT MONITOR

        Refer to "Diagnostic Trouble Code (DTC)" for detailed operation procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) .

      • General scan tool

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

    Is DTC U0077 displayed? (Current malfunction)

    Yes:  Replace the ECM. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>ENGINE CONTROL MODULE (ECM) .

    No:  Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.

    NOTE: In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause. Clear the memory using the Subaru Select Monitor or general scan tool.
    • SUBARU SELECT MONITOR

      For detailed procedures, refer to "Clear memory". Ref. to COMMON (diag)>CLEAR MEMORY .

    • General scan tool

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

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