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>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DTC. 

   Check for DTC.

   NOTE:

   • Subaru Select Monitor

     Refer to "Read Diagnostic Trouble Code (DTC)" for detailed operation procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>READ 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 Mode". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>CLEAR MEMORY MODE

   • General scan tool

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

2. CHECK BETWEEN ECM CONNECTOR AND BATTERY SENSOR CONNECTOR, AND BETWEEN ECM CONNECTOR AND ACTIVE GRILLE SHUTTER CONNECTOR. 
   1. Turn the ignition switch to OFF.
   2. Disconnect the ECM connector.
   3. Disconnect the battery sensor connector.
   4. Disconnect the connector of the active grille shutter.
   5. Measure the resistance of terminals between ECM connector and battery sensor connector, as well as ECM connector and active grille shutter connector.

      Connector & terminal 

      (B134) No. 55 - (B78) No. 2:

      (B134) No. 55 - (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 of harness between ECM connector and battery sensor connector, and between ECM connector and active grille shutter connector
   • Poor contact of coupling connector
3. CHECK WIRING HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR CONNECTOR, AND BETWEEN ECM CONNECTOR AND ACTIVE GRILLE SHUTTER CONNECTOR. 

   Measure the resistance between ECM connector and chassis ground.

   Connector & terminal 

   (B134) No. 55 - Chassis ground:

   Is the resistance 1 MΩ or more?

   Yes:  Go to  4

   No:  Repair the short circuit to ground in harness between ECM connector and battery sensor connector, as well as ECM connector and active grille shutter connector.

4. CHECK WIRING HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR CONNECTOR, AND BETWEEN ECM CONNECTOR AND ACTIVE GRILLE SHUTTER CONNECTOR. 
   1. Turn the ignition switch to ON.
   2. Measure the voltage between ECM connector and chassis ground.

      Connector & terminal 

      (B134) No. 55 - Chassis ground:

   Is the voltage less than 1 V?

   Yes:  Go to  5

   No:  Repair the short circuit to ground in harness between ECM connector and battery sensor connector, as well as ECM connector and active grille shutter connector.

5. CHECK DTC. 
   1. Turn the ignition switch to OFF.
   2. Connect the battery sensor connector.
   3. Connect the ECM connector.
   4. Clear the memory using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Clear Memory Mode>OPERATION
   5. Check for DTC.
      NOTE:

      • Subaru Select Monitor

        Refer to "Read Diagnostic Trouble Code (DTC)" for detailed operation procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>READ 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 Mode". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>CLEAR MEMORY MODE