DTC U1678: Lin Communication Bus 4 Error Body Control Module

DTC detecting condition  :

Integrated unit communication is shut down because of LIN error.

Trouble symptom  :

LIN communication is not normal.

Wiring diagram  :

Clearance light and illumination light system Ref. to WIRING SYSTEM>CLEARANCE LIGHT AND ILLUMINATION LIGHT SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DTC  .
   1. Turn the ignition switch to OFF.
   2. Turn the ignition switch to ON.
   3. Wait for 10 seconds.
   4. Read the DTC of [Body Control] using the Subaru Select Monitor. Ref. to BODY CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC U1678 displayed? (Current code)

   Yes:  Go to  2.

   No:  Even if DTC is displayed, 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.
2. CHECK BODY INTEGRATED UNIT CUSTOMIZATION  .

   Using Subaru Select Monitor, make sure the [Customize] setting values of [Body Control] correspond to the vehicle.

   Is the setting value of [Light LIN setting] equal to [Exist]?

   Yes:  Go to  3.

   No:  Change the setting value to [ON].

3. CHECK CONTROL MODULE  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from front combination light LH.
   3. Turn the ignition switch to ON.
   4. Wait for 10 seconds.
   5. Read the DTC of [Body Control] using the Subaru Select Monitor. Ref. to BODY CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC U1678 displayed? (Current code)

   Yes:  Go to  4.

   No:  Replace the front combination light LH. Ref. to LIGHTING SYSTEM>HEADLIGHT ASSEMBLY .

4. CHECK CONTROL MODULE  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from the front combination light RH. (With the front combination light LH connector disconnected)
   3. Turn the ignition switch to ON.
   4. Wait for 10 seconds.
   5. Read the DTC of [Body Control] using the Subaru Select Monitor. Ref. to BODY CONTROL (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

   Is DTC U1678 displayed? (Current code)

   Yes:  Go to  5.

   No:  Replace the front combination light RH. Ref. to LIGHTING SYSTEM>HEADLIGHT ASSEMBLY .

5. CHECK HARNESS (OPEN CIRCUIT)  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from body integrated unit.
   3. Using a tester, measure the resistance between the body integrated unit connector and the front combination light LH/RH connectors.

      Connector & terminal 

      (B73) No. 20 - (B121) No. 2:

      (B73) No. 20 - (B122) No. 2:

   Is the resistance less than 10 Ω?

   Yes:  Go to  6.

   No:  Repair the open circuit of harness.

6. CHECK HARNESS (SHORT CIRCUIT TO POWER SUPPLY)  .

   Using a tester, measure the voltage between the front combination light LH/RH connectors and chassis ground.

   Connector & terminal 

   (B121) No. 2 (+) - Chassis ground (-):

   (B122) No. 2 (+) - Chassis ground (-):

   Is the voltage less than 1 V?

   Yes:  Go to  7.

   No:  Repair the short circuit to power supply in harness.

7. CHECK HARNESS (GROUND SHORT CIRCUIT)  .

   Using a tester, measure the resistance between the front combination light LH/RH connectors and chassis ground.

   Connector & terminal 

   (B121) No. 2 - Chassis ground:

   (B122) No. 2 - Chassis ground:

   Is the resistance 1 MΩ or more?

   Yes:  Replace the body integrated unit. Ref. to BODY CONTROL/COMMUNICATION SYSTEM>BODY INTEGRATED UNIT .

   No:  Repair the ground short circuit of harness.