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  .

   Read the engine DTC using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H6DO)>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) (H6DO)>CLEAR MEMORY MODE .

2. CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR (OPEN CIRCUIT)  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from ECM.
   3. Disconnect the connector from battery sensor.
   4. Measure the resistance of harness between ECM connector and battery sensor connector.

   Connector & terminal 

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

   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
   • Poor contact of coupling connector
3. CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR (SHORTED TO GROUND)  .

   Measure the resistance between ECM connector and chassis ground.

   Connector & terminal 

   (B134) No. 55 - Chassis ground:

   Is the resistance 1 MΩ?

   Yes: Go to  4 .

   No: Repair the short circuit to ground in harness between ECM connector and battery sensor connector.

4. CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR 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 the harness between ECM connector and generator connector.

5. CHECK DTC  .
   1. Turn the ignition switch to OFF.
   2. Connect the connector to ECM.
   3. Connect the connector to the battery sensor.
   4. Clear the memory using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H6DO)>CLEAR MEMORY MODE .
   5. Read the engine DTC using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H6DO)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC U0077 displayed? (Current malfunction)

   Yes: Replace the ECM. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H6DO)>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.