What it means
Brake Booster Motor B Position Sensor Supply Circuit Range/Performance: the signal is implausible or outside the valid range. 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
Motor B's position sensor helps the electric brake booster control braking assist. The brake control module monitors the sensor supply circuit and sets C05BF when its behavior is implausible or outside the valid range, even if the circuit is not simply open or shorted.
Common causes
Most likely first.
- Connector with high resistance
- Sensor element aged beyond calibration window
- Sensor mounting loose - signal damping
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, freeze-frame data, and live readings from motor B's position-sensor supply to identify when the value becomes implausible.
- Inspect the sensor connector and harness for loose or corroded contacts that could create high resistance.
- Check that the motor B position sensor is securely mounted and has not shifted or loosened.
- Test the supply circuit and connections for intermittent resistance or damage, using the manufacturer's procedure and specification.
- If the circuit and mounting are sound, assess the sensor and module for an internal fault, then clear the code and verify stable readings.
Common fixes
- Secure the motor B position sensor if its mounting is loose.
- Clean or replace high-resistance connector terminals.
- Repair damaged wiring or connections in the sensor supply circuit.
- Replace the sensor or repair the module if testing confirms it is producing an out-of-range supply condition.
Can I keep driving?
Brake assist may be reduced and stability control may be disabled, so avoid driving until brake operation is confirmed normal. Stop safely and arrange recovery if the pedal feels unusually hard or braking changes.
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.