Subaru Legacy BN/BS (2015-2019) Service Manual: Dtc u0077 lin communication bus "ecm/pcm" off

Subaru Legacy BN/BS (2015-2019) Service Manual / Engine (diagnostics)(h6do) / Diagnostic procedure with diagnostic trouble code (dtc) / Dtc u0077 lin communication bus "ecm/pcm" off

DTC DETECTING CONDITION:

Immediately at fault recognition

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode Clear Memory Mode > OPERATION, and Inspection Mode Inspection Mode > PROCEDURE.

WIRING DIAGRAM:

• Engine Electrical System ENGINE TYPE EZ (WITHOUT PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

• Engine Electrical System ENGINE TYPE EZ (WITH PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

STEP	CHECK	YES	NO
1.CHECK DTC. Check for DTC. Read Diagnostic Trouble Code (DTC)	Is U0077 displayed in «Present fault»-	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC U0077 LIN COMMUNICATION BUS "ECM/PCM" OFF	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-	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC U0077 LIN COMMUNICATION BUS "ECM/PCM" OFF	Connect the ECM connector correctly.
3.CHECK HARNESS BETWEEN BATTERY AND BATTERY SENSOR CONNECTOR. 1) Turn the ignition switch to OFF. 2) Disconnect the ground terminal from battery sensor. NOTE 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 --	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC U0077 LIN COMMUNICATION BUS "ECM/PCM" OFF	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 M--	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC U0077 LIN COMMUNICATION BUS "ECM/PCM" OFF	Repair the short circuit to ground in harness between ECM connector and battery sensor connector.
5.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 (B134) No. 55 — (B78) No. 2:	Is the resistance less than 1 --	Check for the poor contact of ECM connector or battery sensor connector. Repair or replace if defective. Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC U0077 LIN COMMUNICATION BUS "ECM/PCM" OFF	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. 1) Connect ECM connector and battery sensor connector. 2) Check for DTC. Read Diagnostic Trouble Code (DTC)	Is U0077 displayed in «Present fault»-	ECM is faulty. Replace the ECM. Engine Control Module (ECM)	The circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.

1. OUTLINE OF DIAGNOSIS

Detect malfunction of LIN communication.

Judge as NG when the data sent/received via LIN communication is abnormal.

2. COMPONENT DESCRIPTION

LIN communication is used to send/receive data between ECM and battery sensor.

3. EXECUTION CONDITION

Secondary Parameters	Execution condition
Battery voltage	≥ 10.9 V

4. GENERAL DRIVING CYCLE

Always perform the diagnosis continuously.

5. DIAGNOSTIC METHOD

If the duration of time while the following conditions are met is longer than the time indicated, judge as NG.

Judgment Value

Malfunction Criteria

Threshold Value

Data sent/received with control modules connected to LIN

No change

Checksum error

Time Needed for Diagnosis: 1280 ms

Malfunction Indicator Light Illumination: Illuminates as soon as a malfunction occurs.

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