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

DTC detecting condition: 

Immediately at fault recognition

Wiring diagram: 

Charging System Ref. to WIRING SYSTEM>CHARGING SYSTEM .

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DTC  .

   Read the engine DTC using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

   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. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>CLEAR MEMORY MODE .

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

      Connector & terminal 

      (B134) No. 55 - (B471) No. 1:

      (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 generator connector
   • Open circuit in harness between ECM connector and battery sensor connector
   • Open circuit of harness between ECM connector and active grille shutter connector
   • Poor contact of coupling connector
3. CHECK HARNESS BETWEEN ECM AND GENERATOR CONNECTOR, BATTERY SENSOR CONNECTOR AND ACTIVE GRILLE SHUTTER CONNECTOR (SHORT TO GROUND)  .

   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 ground short circuit of harness.

   NOTE: In this case, repair the following item:

   • Short circuit to ground in harness between ECM connector and generator connector
   • Short circuit to ground in harness between ECM connector and battery sensor connector
   • Short circuit to ground in harness between ECM connector and active grille shutter connector
4. CHECK HARNESS BETWEEN ECM AND GENERATOR CONNECTOR, BATTERY SENSOR CONNECTOR AND ACTIVE GRILLE SHUTTER CONNECTOR (SHORTED TO POWER SUPPLY)  .
   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 power supply in harness.

   NOTE: In this case, repair the following item:

   • Short circuit to power supply in harness between ECM connector and generator connector
   • 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 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 battery sensor.
   4. Connect the connector to the generator.
   5. Connect the connector to ECM.
   6. Clear the memory using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>CLEAR MEMORY MODE .
   7. Read the engine DTC using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC U0077 displayed? (Current malfunction)

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

   No: The circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.