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

DTC detecting condition:

Immediately at fault recognition

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode Ref. to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>Clear Memory Mode>OPERATION , and inspection mode Ref. to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>Inspection Mode>PROCEDURE .

Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram:

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

CHECK DTC .
Read DTC using the Subaru Select Monitor or a general scan tool. Ref. to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>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.
CHECK ECM CONNECTOR .
Check the connecting condition of ECM connector.

Is the ECM connector correctly connected?

Yes: Go to 3

No: Connect the ECM connector correctly.
CHECK HARNESS BETWEEN BATTERY AND BATTERY SENSOR CONNECTOR .
1. Turn the ignition switch to OFF.
2. Disconnect the ground terminal from battery sensor. Ref. to NOTE>NOTE>BATTERY .
3. Disconnect the connector from battery sensor.
4. Disconnect the positive terminal of battery.
5. Measure the resistance of the harness between battery positive cable terminal and battery sensor connector.
  Connector & terminal
  
  Positive cable terminal - (B9) No. 1:
Is the resistance less than 1 Ω?

Yes: Go to 4

No: Repair the open circuit in harness between battery positive cable terminal and battery sensor connector.
CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR .
1. Disconnect the connector from ECM.
2. Measure the resistance of harness between ECM connector and battery sensor connector.
  Connector & terminal
  
  (E158) No. 3 - (B9) No. 2:
Is the resistance less than 1 Ω?

Yes: Repair the poor contact of ECM connector or battery sensor connector.

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