LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Subaru >> 2025 >> Outback Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System (2.5L) (Diagnostics) (4 Of 4) >> Diagnostic Procedure with Diagnostic Trouble Code (DTC) >> DTC U0077: Lin Communication Bus "ECM/Pcm" Off >> Notes
April 5, 2026: LEMON Manuals is launched! Read the announcement.

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

DTC detecting condition: 

Immediately at fault recognition

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM .

G00687005Courtesy of SUBARU OF AMERICA, INC.
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 .
  1. CHECK BATTERY SENSOR  .
    1. Turn the ignition switch to ON.
    2. Disconnect the connector from battery sensor.
    3. Turn the ignition switch to ON (engine OFF).
    4. Wait for 5 seconds.
    5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    Is DTC U0077 displayed? (Current code)

    Yes:  Go to  2.

    No:  Replace the battery sensor. Ref. to STARTING/CHARGING SYSTEMS(H4DO)>Battery Sensor

  2. CHECK GENERATOR  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from the generator.
    3. Turn the ignition switch to ON (engine OFF).
    4. Wait for 5 seconds.
    5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    Is DTC U0077 displayed? (Current code)

    Yes:  Go to  3.

    No:  Replace the generator. Ref. to STARTING/CHARGING SYSTEMS(H4DO)>Generator .

  3. CHECK ACTIVE GRILLE SHUTTER  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from the active grille shutter.
    3. Turn the ignition switch to ON (engine OFF).
    4. Wait for 5 seconds.
    5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    Is DTC U0077 displayed? (Current code)

    Yes:  Go to  4.

    No:  Replace the active grille shutter. Ref. to STARTING/CHARGING SYSTEMS(H4DO)>Auto Start Stop DC/DC Converter>REMOVAL

  4. CHECK DC/DC CONVERTER 1  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from the DC/DC converter 1.
    3. Turn the ignition switch to ON (engine OFF).
    4. Wait for 5 seconds.
    5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    If the audio amplifier is equipped and DTC U0077 is displayed, go to A. (Current code)

    If the audio amplifier is not equipped and DTC U0077 is displayed, go to B. (Current code)

    If DTC U0077 is not displayed, go to C.

    A  Go to  5.

    B  Go to  6

    C  Replace the DC/DC converter 1. Ref. to STARTING/CHARGING SYSTEMS(H4DO)>Auto Start Stop DC/DC Converter

  5. CHECK DC/DC CONVERTER 2 (WITH AUDIO AMPLIFIER)  .
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from the DC/DC converter 2.
    3. Turn the ignition switch to ON (engine OFF).
    4. Wait for 5 seconds.
    5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    Is DTC U0077 displayed? (Current code)

    Yes:  Go to  6.

    No:  Replace the DC/DC converter 2. Ref. to STARTING/CHARGING SYSTEMS(H4DO)>Auto Start Stop DC/DC Converter

  6. CHECK WIRING HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR, GENERATOR, ACTIVE GRILLE SHUTTER AND DC/DC CONVERTER CONNECTOR (OPEN) 
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from ECM.
    3. Measure the resistance between the following terminals.

      4. Connector & terminal 

      (B62) No. 17 - (B91) No. 3:

      (B62) No. 17 - (Bt52) No. 1:

      (B62) No. 17 - (B136) No. 3:

      (B62) No. 17 - (B11) No. 9:

      Model with audio amplifier; (B62) No. 17 - (i16) No. 1:

      Is the resistance less than 1 Ω?

      Yes:  Go to  7

      No:  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
    • Open circuit in harness between ECM connector and DC/DC converter 1 connector
    • Open circuit in harness between ECM connector and DC/DC converter 2 connector (model with audio amplifier)
    • Poor contact of coupling connector
  7. CHECK WIRING HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR, GENERATOR, ACTIVE GRILLE SHUTTER AND DC/DC CONVERTER CONNECTOR (SHORT TO GROUND)  .

    Measure the resistance between the ECM connector and engine ground.

    Connector & terminal 

    (B62) No. 17 - Engine ground:

    Is the resistance 1 MΩ or more?

    Yes:  Go to  8

    No:  Repair the following item.

    • Short circuit to ground in harness between ECM connector and battery sensor connector
    • Short circuit to ground in harness between ECM connector and generator connector
    • Short circuit to ground in harness between ECM connector and active grille shutter connector
    • Short circuit to ground in harness between ECM connector and DC/DC converter 1 connector
    • Short circuit to ground in harness between ECM connector and DC/DC converter 2 connector (model with audio amplifier)
  8. CHECK WIRING HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR, GENERATOR, ACTIVE GRILLE SHUTTER AND DC/DC CONVERTER CONNECTOR (SHORT TO POWER SUPPLY)  .

    Measure the resistance between the following terminals.

    Connector & terminal 

    (B62) No. 17 or (B91) No. 3 - Other terminals:

    (B62) No. 17 or (Bt52) No. 1 - Other terminals:

    (B62) No. 17 or (B136) No. 3 - Other terminals:

    (B62) No. 17 or (B11) No. 9 - Other terminals:

    Models with audio amplifier; (B62) No. 17 or (i16) No. 1 - Other terminals:

    Is the resistance less than 1 Ω?

    Yes:  Go to  9

    No:  Repair the following item.

    • Short circuit to power supply in harness between ECM connector and battery sensor connector
    • Short circuit to power supply in harness between ECM connector and generator connector
    • Short circuit to power supply in harness between ECM connector and active grille shutter connector
    • Short circuit to power supply in harness between ECM connector and DC/DC converter 1 connector
    • Short circuit to power supply in harness between ECM connector and DC/DC converter 2 connector (model with audio amplifier)
  9. CHECK DTC  .
    1. Turn the ignition switch to OFF.
    2. Connect all connectors.
    3. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. COMMON (DIAGNOSTICS)>Clear memory
    4. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>Diagnostic Trouble Code (DTC)

    Is DTC U0077 displayed? (Current code)

    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.
scientia non olet · About LEMON Manuals