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

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

1. -

DTC detecting condition:

Immediately at fault recognition

Wiring diagram:

Engine Electrical System Ref. to WIRING SYSTEM (IMPREZA MODEL)>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM . Ref. to WIRING SYSTEM (CROSSTREK MODEL (EXCEPT FOR HEV))>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

IMPREZA MODEL

Courtesy of SUBARU OF AMERICA, INC.
CROSSTREK MODEL

Courtesy of SUBARU OF AMERICA, INC.

CAUTION: Use the check board when measuring the ECM terminal voltage and resistance. Ref. to ENGINE (DIAGNOSTICS) (H4DO 2.0 (EXCEPT FOR HEV))>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) (H4DO 2.0 (EXCEPT FOR HEV))>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

CHECK DTC .
Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAG)>DIAGNOSTIC TROUBLE CODE (DTC) .

Is DTC U0077 displayed? (Current malfunction)

Yes: Go to 1.

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. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
{CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (OPEN).}
1. Turn the ignition switch to OFF.
2. Disconnect the connector from ECM.
3. Disconnect the connector from battery sensor.
4. Disconnect the connector from the generator.
5. Disconnect the connector from the active grille shutter.
6. Measure the resistance of harness between ECM connector and battery sensor connector, generator connector and active grille shutter connector.
  Connector and terminal
  
  (B134) No. 17 - (B9) No. 3:
  
  (B134) No. 17 - (F26) No. 1:
  
  (B134) No. 17 - (B63) No. 3: Model with active grille shutter
  
  (B134) No. 17 - (B562) No. 9: Model with Auto Start Stop
  
  (B134) No. 17 - (B630) No. 1: Model with power amplifier
  
  Is the resistance less than 1 Ω?
  
  Yes: Go to 3.
  
  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
  - Open circuit of harness between ECM connector and generator connector
  - Open circuit of harness between ECM connector and active grille shutter connector (model with active grille shutter)
  - Open circuit in harness between ECM connector and DC/DC converter 1 connector (model with Auto Start Stop)
  - Open circuit in harness between ECM connector and DC/DC converter 2 connector (model with power amplifier)
  - Poor contact of coupling connector
CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (SHORT TO GROUND) .
Measure the resistance between ECM connector and chassis ground.

Connector and terminal

(B134) No. 17 - Chassis ground:

Is the resistance 1 MΩ or more?

Yes: Go to 4.

No: Repair the ground short circuit of harness.
NOTE:
In this case, 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
- Short circuit to ground in harness between ECM connector and active grille shutter connector (model with active grille shutter)
- Short circuit to ground in harness between ECM connector and DC/DC converter 1 connector (model with Auto Start Stop)
- Short circuit to ground in harness between ECM connector and DC/DC converter 2 connector (model with power amplifier)
CHECK WIRING HARNESS BETWEEN ECM AND BATTERY SENSOR, GENERATOR AND ACTIVE GRILLE SHUTTER (SHORT TO POWER SUPPLY) .
1. Turn the ignition switch to ON.
2. Measure the voltage between ECM connector and chassis ground.
  Connector and terminal
  
  (B134) No. 17 (+) - Chassis ground (-):
  
  Is the voltage less than 1 V?
  
  Yes: Go to 5.
  
  No: Repair the short circuit to power supply in harness.
  NOTE:
  In this case, 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
  - Short circuit to power supply in harness between ECM connector and active grille shutter connector (model with active grille shutter)
  - Short circuit to power supply in harness between ECM connector and DC/DC converter 1 connector (model with Auto Start Stop)
  - Short circuit to power supply in harness between ECM connector and DC/DC converter 2 connector (model with power amplifier)
CHECK DTC .
1. Turn the ignition switch to OFF.
2. Connect the connector to the active grille shutter.
3. Connect the connector to the generator.
4. Connect the connector to the battery sensor.
5. Connect the connector to ECM.
6. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
7. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAG)>DIAGNOSTIC TROUBLE CODE (DTC) .
  Is DTC U0077 displayed? (Current malfunction)
  
  Yes: Replace the ECM. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO (EXCEPT FOR HEV))>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. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (DIAG)>CLEAR MEMORY .