DTC U0077: LIN Communication Bus "ECM/PCM" Off: Notes

-
DTC detecting condition:

Immediately at fault recognition

Wiring diagram:

Engine Electrical System Refer to WIRING SYSTEM (IMPREZA MODEL)>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM . Refer to WIRING SYSTEM (CROSSTREK MODEL (EXCEPT FOR HEV))>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

CAUTION: Use the check board when measuring the ECM terminal voltage and resistance. Refer to ENGINE (DIAGNOSTICS) (H4DO (EXCEPT FOR HEV))>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. Refer to ENGINE (DIAGNOSTICS) (H4DO (EXCEPT FOR HEV))>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

CHECK DTC .
Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Refer to ENGINE (DIAGNOSTICS) (H4DO (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
NOTE:
- SUBARU SELECT MONITOR
  Refer to "Diagnostic Trouble Code (DTC)" for detailed operation procedure. Refer to ENGINE (DIAGNOSTICS) (H4DO) (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
- General scan tool
  For detailed operation procedures, refer to the general scan tool operation instructions.
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". Refer to COMMON (DIAG)>CLEAR MEMORY .
- General scan tool
  For detailed operation procedures, refer to the general scan tool operation instructions.
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: Model with active grille shutter
  
  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 (model with active grille shutter)
  - Poor contact of coupling connector
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:
- 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 (model with active grille shutter)
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:
  - 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 (model with active grille shutter)
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]. Refer to COMMON (DIAG)>CLEAR MEMORY .
7. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Refer to ENGINE (DIAGNOSTICS) (H4DO (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
  NOTE:
  - SUBARU SELECT MONITOR
    Refer to "Diagnostic Trouble Code (DTC)" for detailed operation procedure. Refer to ENGINE (DIAGNOSTICS) (H4DO (EXCEPT FOR HEV))>DIAGNOSTIC TROUBLE CODE (DTC) .
  - General scan tool
    For detailed operation procedures, refer to the general scan tool operation instructions.
  Is DTC U0077 displayed? (Current malfunction)
  
  Yes:
  
  Replace the ECM. Refer to FUEL INJECTION (FUEL SYSTEMS) (H4DO (EXCEPT FOR HEV))>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". Refer to COMMON (DIAG)>CLEAR MEMORY .
  - General scan tool
    For detailed operation procedures, refer to the general scan tool operation instructions.