DTC U115B: Lost Communication With Restraints Control Module Local Bus

Diagnosis start condition:

When ignition switch is ON.

DTC detecting condition:

Malfunction of CAN communication harness between airbag control module and DCM.
Defective airbag control module.
Defective DCM.

Trouble symptom:

Impossible to send CAN data.
RED LED illuminates.

Wiring diagram:

Telematics Ref. to WIRING SYSTEM>TELEMATICS SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

CAUTION:

After the DCM is replaced, registrations of Comm Check and immobilizer system are required. Ref. to ENTERTAINMENT & MONITORING>TELEMATICS SYSTEM>OPERATION>REGISTRATION (COMM CHECK) .
Before performing diagnosis, refer to "CAUTION" in "General Description". Ref. to TELEMATICS SYSTEM (DIAGNOSTICS)>General Description>CAUTION .

CHECK DTC .
1. Turn the ignition switch to ON.
2. Read the DTC of [Telematics] using the Subaru Select Monitor. Ref. to Telematics System (Diagnostics)>Diagnostic Trouble Code (DTC) .
Is DTC U115B displayed? (Current malfunction)

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.
CHECK HARNESS (OPEN CIRCUIT) .
1. Turn the ignition switch to OFF.
2. Disconnect the airbag control module connector.
3. Disconnect the DCM connector.
4. Measure the resistance between airbag control module connector and DCM connector.
CAUTION:
To measure the voltage and resistance of airbag system component, be sure to use the specified test harness. Ref. to AIRBAG (DIAGNOSTICS)>GENERAL DESCRIPTION .

Connector and terminal

(8BF) No. 1 - (i105) No. 6:

(8BF) No. 2 - (i105) No. 16:

Is the resistance 10 Ω or less?

Yes : Go to 3 .

No : Repair or replace the open circuit of harness.
CHECK HARNESS (GROUND SHORT CIRCUIT) .
Measure the resistance between DCM connector and chassis ground.

Connector and terminal

(i105) No. 6 - Chassis ground:

(i105) No. 16 - Chassis ground:

Is the resistance 1 MΩ or more?

Yes : Go to 4 .

No : Repair or replace the short circuit of the harness.
CHECK THE CONNECTOR (SHORT CIRCUIT TO POWER SUPPLY) .
1. Turn the ignition switch to ON.
2. Measure the voltage between DCM connector and chassis ground.
  Connector and terminal
  
  (i105) No. 6 (+) - Chassis ground (-):
  
  (i105) No. 16 (+) - Chassis ground (-):
Is the voltage less than 5 V?

Yes : Go to 5 .

No : Repair or replace the short circuit of the harness.
CHECK DCM (INTERNAL RESISTANCE) .
1. Turn the ignition switch to OFF.
2. Measure the internal resistance of DCM.
Terminals

(i105) No. 6 - (i105) No. 16: (internal resistance of DCM)

Is the resistance 114 - 126 Ω?

Yes : Go to 6 .

No : Replace the DCM. Ref. to ENTERTAINMENT & MONITORING>DATA COMMUNICATION MODULE .
CHECK AIRBAG CONTROL MODULE (INTERNAL RESISTANCE) .
1. Connect the airbag control module connector.
2. Measure the resistance between DCM connectors. (Measure the internal resistance of airbag control module from DCM connectors)
Terminals

(i105) No. 6 - (i105) No. 16: (internal resistance of airbag control module)

Is the resistance 114 - 126 Ω?

Yes : Go to 7 .

No : Replace the airbag control module. Ref. to AIRBAG SYSTEM>AIRBAG CONTROL MODULE .
CHECK DCM .
1. Restore the vehicle to its original state.
2. Turn the ignition switch to ON.
3. Using the Subaru Select Monitor, perform the clear memory of [Telematics]. Ref. to COMMON (DIAG)>CLEAR MEMORY .
4. Turn the ignition switch to OFF.
5. Turn the ignition switch to ON.
6. Wait for 1 minute.
7. Read the DTC of [Telematics] using the Subaru Select Monitor. Ref. to Telematics System (Diagnostics)>Diagnostic Trouble Code (DTC) .
Is DTC U115B displayed? (Current malfunction)

Yes : Replace the DCM. Ref. to ENTERTAINMENT & MONITORING>DATA COMMUNICATION MODULE .

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.