DTC B178D: Front Left Occupant Detection Sensor Lost Communication

Diagnosis start condition: 

When the ignition voltage is 10 - 16 V.

DTC detecting condition: 

• Open or short circuit of the occupant detection sensor harness
• Occupant detection sensor malfunction

Wiring diagram: 

Occupant detection system Ref. to WIRING SYSTEM>OCCUPANT DETECTION SYSTEM>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DTC  .

   Read the DTC of [Airbag] using the Subaru Select Monitor. Ref. to AIRBAG (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) 

   Is any of DTC B1655 or [Airbag ECU Malfunction] displayed? (Current code)

   Yes  : Perform the diagnosis for the displayed DTC B1655 or [Airbag ECU Malfunction]. Ref. to AIRBAG (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) 

   No  : Go to  2.

2. CHECK SEAT HARNESS (OPEN CIRCUIT)  .
   1. Turn the ignition switch to OFF, and wait for 60 seconds or more after disconnecting the ground terminal from the battery sensor. Ref. to REPAIR CONTENTS>NOTE>BATTERY
   2. Disconnect the connector (AB93) from seat belt pretensioner RH.
   3. Disconnect the connector (AB95) from the lap seat belt pretensioner RH.
   4. Disconnect the connector (AB99) from rear seat belt pretensioner RH.
   5. Disconnect the connector (AB87) from side airbag module RH.
   6. Disconnect the connector (AB91) from curtain airbag module RH.
   7. Disconnect the connectors (AB41, AB42, AB43) from airbag control module.
   8. Connect the connector (1BF) in the test harness BF to the connectors (AB41, AB42, AB43).
   9. Disconnect connector (AB68) from the load sensor FL.
   10. Using a tester, measure the resistance between the connector (4BF) in test harness BF and the load sensor FL connector (AB68).
       CAUTION:

       Connect the terminal in test harness BC to connector (AB68) terminal, and apply the tester to test harness BC. 

       Connector & terminal 

       (4BF) No. 10 - (AB68) No. 1:

       (4BF) No. 9 - (AB68) No. 2:

       (4BF) No. 15 - (AB68) No. 3:

   Is the resistance less than 10 Ω?

   Yes  : Go to  3.

   No  : Check the seat harness, and if any fault is found, replace the seat harness. If there is no fault, replace the airbag rear harness together with the body harness.

3. CHECK SEAT HARNESS (SHORTED BETWEEN LINES AND SHORTED TO GROUND)  .

   Measure the resistance between connector (4BF) in the test harness BF and chassis ground, and the resistance between connector (4BF) terminals in the test harness BF.

   Connector & terminal 

   (4BF) No. 10 - All terminals other than (4BF) No. 10:

   (4BF) No. 9 - All terminals other than (4BF) No. 9:

   (4BF) No. 15 - All terminals other than (4BF) No. 15:

   (4BF) No. 10 - Chassis ground:

   (4BF) No. 9 - Chassis ground:

   (4BF) No. 15 - Chassis ground:

   Is the resistance 1 MΩ or more?

   Yes  : Go to  4.

   No  : Check the seat harness, and if any fault is found, replace the seat harness. If there is no fault, replace the airbag rear harness together with the body harness.

4. CHECK SEAT HARNESS (BATTERY SHORT)  .
   1. Install the ground terminal to the battery sensor and turn the ignition switch to ON. Ref. to REPAIR CONTENTS>NOTE>BATTERY
   2. Measure the voltage between connector (4BF) in the test harness BF and chassis ground.

      Connector & terminal 

      (4BF) No. 10 (+) - Chassis ground (-):

      (4BF) No. 9 (+) - Chassis ground (-):

      (4BF) No. 15 (+) - Chassis ground (-):

   Is the voltage less than 1 V?

   Yes  : Go to  5.

   No  : Check the seat harness, and if any fault is found, replace the seat harness. If there is no fault, replace the airbag rear harness together with the body harness.

5. CHECK CONNECTOR  .
   1. Turn the ignition switch to OFF, and wait for 60 seconds or more after disconnecting the ground terminal from the battery sensor. Ref. to REPAIR CONTENTS>NOTE>BATTERY
   2. Check for poor contact in airbag control module connector and load sensor connector.
   3. Disconnect the test harness BF from the connectors (AB41, AB42, AB43) and connect all the connectors.
   4. Read the DTC of [Airbag] using the Subaru Select Monitor. Ref. to AIRBAG (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) 

   Is DTC B178D displayed? (Current code)

   Yes  : Replace the cushion frame assembly. Ref. to SEATS>FRONT SEAT

   No  : Finish the diagnosis.