Faulty brake booster motor position sensor; Damaged or corroded wiring in the sensor supply circuit; Loose or corroded electrical connector at the sensor or control module; Failed brake booster motor assembly affecting sensor supply; Control module software or internal fault misreading the supply voltage; Poor ground connection shared with the sensor circuit; Aftermarket or incorrect sensor installed previously

DTC codes / C05xx · Brakes, steering and suspension / C05BF

C05BFBrake Booster Motor B Position Sensor Supply Circuit Range/Performance

This code indicates that the brake booster motor's position sensor supply circuit is not delivering the expected voltage range or performance, so the control module cannot trust the sensor's feedback. The computer monitors the supply voltage to the position sensor and sets this code when the signal falls outside the manufacturer's specified range for a calibrated time.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaBrakes, steering and suspension

Symptoms you may notice

Illumination of the brake system warning light or a generic check engine light
Often no symptoms besides the warning light, especially if the fault is intermittent
Reduced or inconsistent brake assist, requiring more pedal effort
A soft or spongy brake pedal feel during normal driving
Unusual noises from the brake booster area when the brake pedal is pressed
The vehicle may enter a limited-power or limp mode in some applications

Common causes, most likely first

Faulty brake booster motor position sensor
Damaged or corroded wiring in the sensor supply circuit
Loose or corroded electrical connector at the sensor or control module
Failed brake booster motor assembly affecting sensor supply
Control module software or internal fault misreading the supply voltage
Poor ground connection shared with the sensor circuit
Aftermarket or incorrect sensor installed previously

How to diagnose and fix it

Inspect the sensor connector and wiring for corrosion, chafing, or loose pins
Measure the sensor supply voltage with a multimeter and compare to specification
Check the ground circuit for excessive resistance using a voltage drop test
Clear the code and test-drive to see if it returns immediately or intermittently
If voltage is out of range, back-probe the sensor signal wire to confirm sensor operation
Replace the brake booster motor position sensor if supply and ground are correct but signal is erratic
Reprogram or replace the control module only after all wiring and sensor checks pass

What the supply circuit code really tells you

A supply circuit range code often points to a voltage drop in the wiring rather than a failed sensor, so always load-test the circuit before condemning parts.
Intermittent codes can be caused by a connector that only loses contact when the engine bay is hot or during vibration, so wiggle-test the harness while monitoring live data.
Some vehicles share the sensor supply with other sensors, and a fault in one can pull down the reference voltage for the booster sensor and set this code.
A weak or failing brake booster motor can draw excessive current and momentarily drop the sensor supply voltage, mimicking a wiring fault.
This code does not always mean the booster motor is failing; the position sensor supply circuit is a separate sub-circuit that can fail independently.

Can I keep driving with C05BF?

If the brake pedal feels normal and the warning light is the only symptom, you can drive cautiously to a repair shop, but avoid heavy braking or towing. If brake assist is noticeably reduced or the pedal feels strange, stop driving and have the vehicle towed to prevent a safety risk.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.