DTC B1788: Occupant Detection Sensor Power Supply

Diagnosis start condition:

When the ignition voltage is 10 - 16 V.

DTC detecting condition:

Open or short circuit of the occupant detection sensor harness
Airbag control module malfunction.
Occupant detection sensor malfunction

Trouble symptom:

Occupant judgment of airbag control module stops.

Failure detection pattern:

Detection by a single system (Primary Failure)

Warning light/indicator light:

Airbag warning light◦

Wiring diagram:

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

Courtesy of SUBARU OF AMERICA, INC.

CAUTION: Before performing diagnosis, refer to "CAUTION" in "General Description". Ref. to AIRBAG (DIAGNOSTICS)>GENERAL DESCRIPTION>CAUTION .

NOTE: Before starting work, prepare TEST HARNESS BC (98299FL030).

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

Is any of DTC B178D, B178E, B178F, B1790, B1791, B1792, B1793 or B1794 displayed? (Current code)

Yes: Perform the diagnosis for the displayed DTCs. Ref. to AIRBAG (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .

No: GO TO 2.
CHECK AIRBAG REAR HARNESS (SHORTED TO GROUND) .
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 (AB99) from rear seat belt pretensioner RH.
4. Disconnect the connector (AB89) from seat cushion airbag module RH.
5. Disconnect the connector (AB87) of 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. Using a tester, measure the resistance between connector (4BF) in the test harness BF and chassis ground, and between connector (4BF) terminals in the test harness BF.
  Connector & terminal
  
  (4BF) No. 10 - Chassis ground:
  
  (4BF) No. 10 - (4BF) No. 15:
NOTE: Connect the tester positive terminal to the IGN side ((4BF) No. 10).
Connect the tester negative terminal to the GND side (chassis ground or (4BF) No. 15).
Reverse polarity will result in incorrect values.

Is the resistance 1 MΩ or more?

Yes: GO TO 4.

No: GO TO 3.
CHECK SEAT HARNESS (GROUND SHORT CIRCUIT) .
1. Disconnect the connector (St1) from airbag rear harness.
2. Using a probe, measure the resistance between the terminals of connector (St1). Ref. to AIRBAG (DIAGNOSTICS)>GENERAL DESCRIPTION>PREPARATION TOOL>HOW TO USE PROBE .
  CAUTION: When measuring the resistance, make sure that the probe is inserted from the back side (harness side) of the connector. Also, do not insert the probe forcibly.
  
  Connector & terminal
  
  (St1) No. 9 - Chassis ground:
  
  (St1) No. 9 - (St1) No. 18:
Is the resistance 1 MΩ or more?

Yes: Replace the airbag rear harness along with body harness.

No: GO TO 8.
CHECK AIRBAG REAR HARNESS (OPEN) .
1. Disconnect connector (AB67, AB68, AB69, AB70) from the load sensor.
2. Using a tester, measure the resistance between the connector (4BF) in test harness BF and the load sensor connector (AB67, AB68, AB69, AB70).
  CAUTION: Connect the terminal in test harness BC to connector (AB67, AB68, AB69, AB70) terminal, and apply the tester to test harness BC.
  
  Connector & terminal
  
  (4BF) No. 10 - (AB67) No. 1:
  
  (4BF) No. 10 - (AB68) No. 1:
  
  (4BF) No. 10 - (AB69) No. 1:
  
  (4BF) No. 10 - (AB70) No. 1:
Is the resistance less than 10 Ω?

Yes: GO TO 6.

No: GO TO 5.
CHECK SEAT HARNESS (OPEN CIRCUIT) .
1. Disconnect the connector (St1) from airbag rear harness.
2. Using a probe, measure the resistance between the connector (St1) and the load sensor connector (AB67, AB68, AB69, AB70). Ref. to AIRBAG (DIAGNOSTICS)>GENERAL DESCRIPTION>PREPARATION TOOL>HOW TO USE PROBE .
  CAUTION:
  - Connect the terminal in test harness BC to connector (AB67, AB68, AB69, AB70) terminal, and apply the tester to test harness BC.
  - When measuring the resistance, make sure that the probe is inserted from the back side (harness side) of the connector. Also, do not insert the probe forcibly.
  Connector & terminal
  
  (St1) No. 9 - (AB67) No. 1:
  
  (St1) No. 9 - (AB68) No. 1:
  
  (St1) No. 9 - (AB69) No. 1:
  
  (St1) No. 9 - (AB70) No. 1:
Is the resistance less than 10 Ω?

Yes: Replace the airbag rear harness along with body harness.

No: GO TO 8.
CHECK AIRBAG REAR 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. Using a tester, measure the voltage between connector (4BF) in test harness BF and chassis ground.
  Connector & terminal
  
  (4BF) No. 10 (+) - Chassis ground (-):
Is the voltage less than 1 V?

Yes: Replace the airbag control module. Ref. to AIRBAG SYSTEM>AIRBAG CONTROL MODULE>REMOVAL .

No: GO TO 7.
CHECK SEAT HARNESS (BATTERY SHORT) .
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 (St1) from airbag rear harness.
3. Install the ground terminal to the battery sensor and turn the ignition switch to ON. Ref. to REPAIR CONTENTS>NOTE>BATTERY .
4. Using a probe, measure the voltage between the connector (St1) and chassis ground. Ref. to AIRBAG (DIAGNOSTICS)>GENERAL DESCRIPTION>PREPARATION TOOL>HOW TO USE PROBE .
  CAUTION: When measuring, make sure that the probe is inserted from the back side (harness side) of the connector. Also, do not insert the probe forcibly.
  
  Connector & terminal
  
  (St1) No. 9 (+) - Chassis ground (-):
Is the voltage less than 1 V?

Yes: Replace the airbag rear harness along with body harness.

No: GO TO 8.
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 all the test harnesses, and connect all the connectors.
4. Install the ground terminal to the battery sensor and turn the ignition switch to ON. Ref. to REPAIR CONTENTS>NOTE>BATTERY .
5. Read the DTC of [Airbag] using the Subaru Select Monitor. Ref. to AIRBAG (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC B1788 displayed? (Current code)

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

No: Finish the diagnosis.