What it means
Brake Booster Motor B Position Sensor Supply Circuit/Open: the circuit is open. The electric brake booster generates the braking assist; a motor or sensor fault can reduce or disable power-assisted braking. Safety-relevant.
At a glance
- Usually lights the ABS or stability-control warning light
- The engine or gearbox may go into limp mode
How it works
The electric brake booster uses motor B and its position sensor to help control braking assist. The brake control module monitors the sensor's supply circuit and sets C05BC when it detects an open, so the supply cannot reach the circuit as expected.
Common causes
Most likely first.
- Supply wire open
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
How to diagnose it
- Scan the brake control module for related codes and freeze-frame data to see when motor B's position-sensor supply fault occurred.
- Inspect the motor B position-sensor connector and harness for damage, a loose connector, or a terminal pushed back into the connector body.
- With the wiring diagram, check continuity through the sensor supply wire and its connections to locate an open.
- Check the connector terminals for secure contact and inspect for a cracked solder joint in the sensor or module if the external wiring checks out.
- After repairs, clear the code and confirm the supply fault does not return while checking brake-system data.
Common fixes
- Reconnect the motor B position-sensor connector and secure any loose harness.
- Repair an open or damaged supply wire.
- Repair or replace a connector terminal that has pushed back or lost contact.
- Repair the cracked solder joint or replace the affected sensor or module if testing confirms an internal fault.
Can I keep driving?
Treat braking assist as potentially reduced and avoid driving until the brake pedal feel and assist are confirmed normal. If the pedal is unusually hard or braking is impaired, stop safely and arrange recovery.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.