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

DTC detecting condition:

Immediately at fault recognition

CAUTION: Use the check board when measuring the ECM terminal voltage and resistance. Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>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. Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

Wiring diagram:

Engine Electrical System Ref. to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM .

Courtesy of SUBARU OF AMERICA, INC.

CHECK BATTERY SENSOR .
1. Turn the ignition switch to OFF.
2. Disconnect the connector from battery sensor.
3. Turn the ignition switch to ON (engine OFF).
4. Wait for 5 seconds.
5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. COMMON (DIAGNOSTICS)>Diagnostic Trouble Code (DTC)
Is DTC U0077 displayed? (Current code)

Yes: Go to 2.

No: Replace the battery sensor. Ref. to STARTING/CHARGING SYSTEMS(H4DOTC)>BATTERY SENSOR .
CHECK GENERATOR .
1. Turn the ignition switch to OFF.
2. Disconnect the connector from the generator.
3. Turn the ignition switch to ON (engine OFF).
4. Wait for 5 seconds.
5. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC U0077 displayed? (Current code)

Yes: Go to 3.

No: Replace the generator. Ref. to STARTING/CHARGING SYSTEMS (H4DOTC)>GENERATOR .
CHECK HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR AND GENERATOR CONNECTOR (OPEN) .
1. Turn the ignition switch to OFF.
2. Disconnect the connector from ECM.
3. Measure the resistance between the following terminals.
  Connector & terminal
  
  (B61) No. 20 - (B91) No. 3:
  
  (B61) No. 20 - (Bt52) No. 1:
Is the resistance less than 1 Ω?

Yes: Go to 4.

No: 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
- Poor contact of coupling connector
CHECK HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR AND GENERATOR CONNECTOR (SHORT TO GROUND) .
Measure the resistance between the ECM connector and engine ground.

Connector & terminal

(B61) No. 20 - Engine ground:

Is the resistance 1 MΩ or more?

Yes: Go to 5.

No: 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
CHECK HARNESS BETWEEN ECM CONNECTOR AND BATTERY SENSOR AND GENERATOR CONNECTOR (SHORT TO POWER SUPPLY) .
Measure the resistance between the following terminals.

Connector & terminal

(B61) No. 20 or (B91) No. 3 - Other terminals:

(B61) No. 20 or (Bt52) No. 1 - Other terminals:

Is the resistance less than 1 Ω?

Yes: Go to 6.

No: 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
CHECK DTC .
1. Turn the ignition switch to OFF.
2. Connect all connectors.
3. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (DIAGNOSTICS)>CLEAR MEMORY .
4. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC U0077 displayed? (Current code)

Yes: Replace the ECM. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>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.