What it means
Brake Booster Motor B Position Sensor Supply 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
The electric brake booster uses motor B and its position sensor to help control braking assist. The brake control module monitors the sensor supply and sets C05BD when the circuit voltage is lower than expected, which can happen if the wire is grounded or a corroded connection restricts it.
Common causes
Most likely first.
- Supply wire shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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 determine when motor B's position-sensor supply voltage fell low.
- Inspect the motor B sensor harness for chafing, crushed sections, or insulation worn through to ground.
- Check the connector for corrosion, moisture, and damaged or loose terminals that could add resistance.
- Using the wiring diagram, measure the supply circuit and check for an unintended short to ground, following the manufacturer's procedure.
- Repair the identified fault, then clear the code and verify the supply reading remains within the manufacturer's specification.
Common fixes
- Clean and properly reconnect corroded motor B sensor terminals, replacing damaged terminals as needed.
- Repair chafed or grounded supply-wire insulation.
- Replace a damaged section of the supply wire or harness.
- Replace the sensor or repair the module only if circuit testing confirms an internal fault.
Can I keep driving?
Brake assist may be reduced and stability control may be unavailable, so avoid normal driving until braking is verified. Stop safely and arrange recovery if the pedal becomes hard or braking response 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.