Damaged, chafed or broken wiring in the sensor supply circuit (open circuit or short to ground or voltage); Corroded, loose or pushed-out pins in the booster motor or control unit connector; Faulty brake booster motor position sensor pulling the supply down or failing internally; Low system voltage or poor earth connection affecting the sensor reference supply; Internal fault in the brake booster control unit supply driver; Outdated or faulty control unit software where the manufacturer has issued a fix for this fault.

C05BC

Brake Booster Motor B Position Sensor Supply Circuit/Open.

Generic
Chassis
Generic code
High severity

How this code is built

C: SystemChassis
0: Defined bySAE, the same on every car
5: AreaChassis area 5
BC: FaultThe specific fault in that area

What it means

C05BC is a generic chassis code meaning the control unit has found a fault in the supply circuit feeding the position sensor of brake booster motor B, or that the supply circuit is open. The module monitors the reference voltage it sends to the sensor and set the code because the voltage was missing, out of range or the circuit was not complete. Without a valid sensor signal, the electric brake booster may reduce or stop its power assistance.

Symptoms

Brake warning light or ABS/ESC warning light on the dashboard
Heavy or stiff brake pedal with reduced power assistance
Brake pedal feel that changes or feels inconsistent
Stability control or other driver assistance systems unavailable
Message on the dash about a brake system fault
Intermittent fault that comes and goes with vibration or damp conditions

Possible causes

Damaged, chafed or broken wiring in the sensor supply circuit (open circuit or short to ground or voltage)likely
Corroded, loose or pushed-out pins in the booster motor or control unit connectorlikely
Faulty brake booster motor position sensor pulling the supply down or failing internallypossible
Low system voltage or poor earth connection affecting the sensor reference supplypossible
Internal fault in the brake booster control unit supply driverpossible
Outdated or faulty control unit software where the manufacturer has issued a fix for this faultrare

How to diagnose

Read all stored codes and freeze frame data, noting whether the fault is current or historic and if other booster or ABS codes are present
Check battery condition and charging voltage, as low voltage can set supply circuit faults
Visually inspect the booster wiring and connectors for chafing, water ingress, corrosion and heat damage
Disconnect the connector and check the pins for tightness, damage and green or white corrosion
Check the sensor supply voltage and earth at the connector against the manufacturer's data for the model
Test the supply wire for continuity and for shorts to ground or voltage, with the battery disconnected as required
Wiggle test the harness with the system powered and monitor live data for signal dropouts
Check for any manufacturer software updates or technical bulletins for the specific model and system

How to fix

Repair or replace the damaged section of wiring in the sensor supply circuit
Clean, repair or replace the corroded or damaged connector terminals
Replace the brake booster assembly or motor position sensor if the sensor is found faulty, following the manufacturer's procedure
Correct battery or earth faults to restore proper system voltage
Replace or repair the brake booster control unit if the supply driver has failed, then carry out any required coding or calibration
Update the control unit software if an applicable manufacturer update addresses the fault

Can I keep driving?

High severity

Avoid driving until the fault is diagnosed, as the brake booster may lose power assistance and braking effort could be much higher. If you must move the car, do so only a short distance at low speed with great care, or arrange recovery.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.