DTC C0021: Brake Booster Performance

DTC detecting condition: 

• When malfunction of the electric brake booster (internal) motor drive circuit/motor is detected.
• When malfunction of the electric brake booster (internal) driver stroke sensor (DTS) is detected.

Trouble symptom: 

• The boost function of electric brake booster does not operate.
• The brake boost function by EyeSight does not operate.

Wiring diagram: 

Electric brake booster system Ref. to WIRING SYSTEM>ELECTRIC BRAKE BOOSTER SYSTEM>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DATA MONITOR  .

   Using the Subaru Select Monitor, read data of [Main Power Supply Voltage] in the [Data Monitor] of [Electric Brake Booster]. Ref. to ELECTRIC BRAKE BOOSTER (DIAGNOSTICS)>DATA MONITOR 

   Is [Main Power Supply Voltage] 10 - 16 V?

   Yes  : Go to  2.

   No  : Check the battery. Ref. to STARTING/CHARGING SYSTEMS (H4DOTC)>BATTERY>INSPECTION

2. CHECK ELECTRIC BRAKE BOOSTER CONTROL MODULE CONNECTOR AND BOOSTER STROKE SENSOR CONNECTOR  .
   1. Turn the ignition switch to OFF.
   2. Check the connecting condition of the electric brake booster control module connector and booster stroke sensor connector.

   Is the check result OK?

   Yes  : Go to  3.

   No  : Repair or replace the connector or harness.

3. CHECK ELECTRIC BRAKE BOOSTER CONTROL MODULE CONNECTOR AND BOOSTER STROKE SENSOR CONNECTOR  .
   1. Disconnect the electric brake booster control module connector and booster stroke sensor connector.
   2. Check the electric brake booster control module connector and booster stroke sensor connector.

   Is there deformation or corrosion in the connector terminals?

   Yes  : Repair or replace the connector.

   No  : Go to  4.

4. CHECK HARNESS  .

   Check the resistance between the electric brake booster control module connector and the booster stroke sensor connector.

   Connector & terminal 

   (B80) No. 22 - (B108) No. 1:

   (B80) No. 2 - (B108) No. 2:

   (B80) No. 23 - (B108) No. 3:

   (B80) No. 8 - (B108) No. 4:

   Is the resistance less than 1 Ω?

   Yes  : Go to  5.

   No  : Repair or replace the harness.

5. RECHECK DTC  .
   1. Connect all connectors.
   2. Using the Subaru Select Monitor, perform the clear memory of [Electric Brake Booster]. Ref. to COMMON (DIAGNOSTICS)>CLEAR MEMORY
   3. Read the DTC of [Electric Brake Booster] using the Subaru Select Monitor. Ref. to ELECTRIC BRAKE BOOSTER (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) 

   Is DTC C0021 displayed?

   Yes  : Replace the electric brake booster control module. Ref. to BRAKE>BRAKE BOOSTER

   No  : Currently, it is normal. Finish the diagnosis.