DTC U1781: Lost Communication With LDM RH 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 HARNESS (OPEN CIRCUIT)  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from body integrated unit.
   3. Disconnect the connector from the front combination light RH.
   4. Using a tester, measure the resistance between the body integrated unit connector and the front combination light RH connector.

      Connector & terminal 

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

   Is the resistance less than 10 Ω?

   Yes:  Go to  4.

   No:  Repair the open circuit of harness.

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

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

   Connector & terminal 

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

   Is the voltage less than 1 V?

   Yes:  Go to  5.

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

5. CHECK HARNESS (GROUND SHORT CIRCUIT)  .

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

   Connector & terminal 

   (B121) 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.