A damaged or chafed wire in the sensor supply circuit, often near the booster motor or along the harness routing.; Corroded or loose electrical connector pins at the brake booster motor position sensor or at the control module.; A failing brake booster motor position sensor that draws excessive current or returns an erratic signal.; High resistance in the supply circuit due to a poor splice or internal wire corrosion.; A faulty control module that cannot provide a stable reference voltage to the sensor.; Water intrusion into the sensor connector or motor housing, causing intermittent shorts or grounds.; Aftermarket accessories or improper previous repairs that tapped into the sensor supply circuit.

DTC codes / C05xx · Brakes, steering and suspension / C05BB

C05BBBrake Booster Motor A Position Sensor Supply Circuit Range/Performance

The C05BB code indicates that the brake booster motor position sensor supply circuit has a range or performance problem, meaning the control module detected an abnormal voltage or signal behavior on the power or reference line feeding that sensor. This sensor reports the position of the brake booster motor, which assists brake application in electro-hydraulic or electronic brake systems, so the module cannot trust the sensor reading and sets this code.

SystemChassis

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaBrakes, steering and suspension

Symptoms you may notice

Illumination of the brake system warning light, often accompanied by a chime or message on the driver information display.
A noticeable reduction in brake assist, requiring higher pedal effort to stop the vehicle.
The brake pedal may feel unusually firm or hard to press, especially during the first application after startup.
Intermittent or complete loss of the brake booster motor function, leading to inconsistent pedal feel.
Often no symptoms besides the warning light, with normal brake operation in many cases.
Activation of a limp-home mode that limits vehicle speed or disables certain driver-assist features.

Common causes, most likely first

A damaged or chafed wire in the sensor supply circuit, often near the booster motor or along the harness routing.
Corroded or loose electrical connector pins at the brake booster motor position sensor or at the control module.
A failing brake booster motor position sensor that draws excessive current or returns an erratic signal.
High resistance in the supply circuit due to a poor splice or internal wire corrosion.
A faulty control module that cannot provide a stable reference voltage to the sensor.
Water intrusion into the sensor connector or motor housing, causing intermittent shorts or grounds.
Aftermarket accessories or improper previous repairs that tapped into the sensor supply circuit.

How to diagnose and fix it

Perform a visual inspection of the brake booster motor position sensor connector and the supply circuit wiring for damage, corrosion, or loose pins.
Use a scan tool to read live data from the position sensor and compare the supply voltage reading with the manufacturer's specification.
Measure the supply voltage at the sensor connector with a digital multimeter, backprobing the terminals with the ignition on and the module connected.
Check for continuity and resistance in the supply wire between the sensor and the control module, looking for excessive resistance or an open circuit.
Inspect the ground circuit for the sensor and motor, ensuring a clean, tight connection to the chassis or module ground.
If the supply circuit and connectors are good, substitute a known-good sensor or perform a resistance test on the sensor itself according to the service information.
Clear the code and perform a road test to verify the repair, monitoring live data for any recurrence of the range or performance fault.

What the supply circuit code really means

A range or performance code for a supply circuit often points to a voltage that is present but outside the expected window, not a complete failure, so intermittent wiring faults are common culprits.
Technicians frequently replace the brake booster motor position sensor when the actual problem is a chafed supply wire near a bracket or a corroded connector pin that only acts up under vibration.
This code can be set by a weak or failing battery or charging system, because low system voltage can cause the module to misinterpret the sensor supply as out of range.
Some vehicles require a relearn or calibration procedure after replacing the sensor or motor, and skipping it can cause the code to return even with good parts.
The code does not always mean the brake booster motor itself is failing; the motor may be fine while the sensor supply circuit is the only issue.

Can I keep driving with C05BB?

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 reduced or the pedal is firm, stop driving and have the vehicle towed, as stopping distance increases significantly.

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.