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Home >> Subaru >> 2017 >> WRX Base >> Repair and Diagnosis (Single Page) >> Engine Performance >> System >> Engine Control System (Diagnostics) (P2101 To U1712) (2.0L) >> Diagnostic Procedure with Diagnostic Trouble Code (DTC) >> DTC U0077 Lin Communication Bus "ECM/Pcm" Off >> Notes

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 OPERATION , Clear Memory Mode.>, and Inspection Mode <Ref. to PROCEDURE , Inspection Mode.>.
  • Use the check board when measuring the ECM terminal voltage and resistance.

WIRING DIAGRAM: 

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

Fig 1: Lin Communication Bus "ECM/PCM" OFF - Wiring Diagram
G11577313Courtesy 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? (Current malfunction) 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. 
  1. Turn the ignition switch to OFF.
  2. Disconnect the ground terminal from battery sensor. <Ref. to BATTERY , NOTE, 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 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. 
  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? Repair the poor contact of ECM connector or battery sensor connector. 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
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