What it means
Brake Booster Motor B Position Sensor Circuit Low: the signal voltage is below the expected 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
Brake booster motor B provides power-assist force, while its position sensor reports motor movement to the brake control module. The module sets C058C when the motor B sensor signal falls below its expected range, which can happen if the signal is pulled toward ground or its connection has excessive resistance.
Common causes
Most likely first.
- Signal wire shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Brake or ABS warning light
- Brake assist may be reduced or unavailable
How to diagnose it
- Capture freeze-frame data and compare motor B position readings with commanded movement to confirm the signal was low when the code set.
- Inspect the motor B sensor connector and harness for corrosion, water intrusion, chafing, or insulation damage that could pull the signal low.
- Check connector terminal condition and fit, paying particular attention to corrosion or weak contact in the signal path.
- Test the motor B signal wire for an unintended short to ground and check circuit continuity using the manufacturer's procedure.
- If the wiring and connector pass, compare sensor output and reference circuits with the manufacturer's specifications to isolate a sensor or module fault.
Common fixes
- Clean and secure corroded or loose motor B sensor connector terminals as appropriate.
- Repair chafed wiring or a signal wire shorted to ground.
- Replace a motor B position sensor that fails its signal checks.
- Repair or replace the brake control module only after the sensor circuit is confirmed sound.
Can I keep driving?
Power assist may be reduced or unavailable, making the brake pedal harder to press and increasing stopping distance. Avoid driving until the booster fault is checked, especially if the brake warning is lit or pedal effort has changed.
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.