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

WARNING: This page is about a different variant/trim than selected.

DTC detecting condition:

Immediately at fault recognition

CAUTION: After servicing or replacing faulty parts, perform Clear Memory Mode Ref. to OPERATION , Clear Memory Mode and Inspection Mode Ref. to PROCEDURE , Inspection Mode.

Wiring diagram:

Engine Electrical System Ref. to WIRING DIAGRAM , Engine Electrical System.

Fig 1: Lost LIN Communication With Battery 1 Monitor Module Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

Step	Check	Yes	No
1 CHECK DTC. Check for DTC. Ref. to READ DIAGNOSTIC TROUBLE CODE (DTC)	Is U0077 displayed in present fault?	Go to step 2	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.
2 CHECK ECM CONNECTOR. Check the connecting condition of ECM connector.	Is the ECM connector correctly connected?	Go to step 3	Connect the ECM connector correctly.
3 CHECK HARNESS BETWEEN BATTERY AND BATTERY SENSOR CONNECTOR. Turn the ignition switch to OFF. Disconnect the ground terminal from battery sensor. Ref. to BATTERY NOTE: Repair Contents. Disconnect the connector from battery sensor. Disconnect the positive terminal of battery. 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?	Go to step 4	Repair the open circuit in harness between battery positive cable terminal and battery sensor connector.
4 CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR. Measure the resistance between ECM connector and chassis ground. Connector & terminal (B134) No. 55 - Chassis ground:	Is the resistance 1 MΩ?	Go to step 5	Repair the short circuit to ground in harness between ECM connector and battery sensor connector.
5 CHECK HARNESS BETWEEN ECM AND BATTERY SENSOR CONNECTOR. Disconnect the connector from ECM. 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?	Check for the poor contact of ECM connector or battery sensor connector. Repair or replace if defective. Go to step 6	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
6 CHECK DTC. Connect the ECM connector. Connect the battery sensor connector. Check for DTC. Ref. to READ DIAGNOSTIC TROUBLE CODE (DTC)	Is U0077 displayed in present fault?	ECM is faulty. Replace the ECM. Ref. to ENGINE CONTROL MODULE (ECM)	The circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.