Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means that the car's computer has detected a problem with the electrical circuit that supplies power to a sensor in the brake system. This sensor helps the car know if the brakes are working properly.
The C05B9 code is triggered when the vehicle's control module detects a low voltage condition in the brake booster motor A position sensor supply circuit.
The brake booster motor A position sensor is responsible for monitoring the position of the brake booster motor, which assists in brake application. A low circuit condition indicates insufficient voltage supply, which can affect the sensor's ability to provide accurate data to the vehicle's control module.
• Illuminated check engine light
• Reduced braking performance
• Unusual brake pedal feel
• Faulty brake booster motor position sensor
• Damaged or corroded wiring or connectors
• Faulty control module
Brake Booster: A device that uses vacuum to increase the force applied to the brakes.
Sensor: A device that detects and measures physical properties and records, indicates, or otherwise responds to it.
• Use an OBD-II scanner to confirm the code and check for other related codes.
• Inspect the wiring and connectors for damage or corrosion.
• Test the brake booster motor position sensor for proper operation.
• Repair or replace damaged wiring or connectors.
• Replace the faulty brake booster motor position sensor.
• Replace or reprogram the control module if necessary.
• OBD-II scanner
• Multimeter
• Basic hand tools
$150 - $400
Moderate
• Chevrolet Malibu
• Buick LaCrosse
• Cadillac CTS
• Regular inspection of brake system components.
• Ensure electrical connections are clean and secure.
• Follow recommended maintenance schedules.
Sensor Issue
• Do not ignore the check engine light; have the vehicle inspected.
• If you notice reduced braking performance, seek professional help immediately.
Check with the vehicle manufacturer for warranty coverage on brake system components.
This code has been associated with issues in vehicles with electronic brake systems, particularly in models with advanced driver assistance systems.
High – Brake system issues can affect vehicle safety and should be addressed promptly.
Driving with a malfunctioning brake system can be dangerous and may violate vehicle safety regulations.
• Vehicle in a safe, stationary position.
• Engine running with all electrical systems active.
The brake booster motor position sensor is typically located near the brake booster assembly in the engine compartment.
Q: Can I drive with the C05B9 code?
A: It's not recommended to drive with this code as it may affect braking performance and vehicle safety.
Q: How do I know if the brake booster motor position sensor is faulty?
A: A professional diagnostic with an OBD-II scanner and multimeter can confirm if the sensor is faulty.
Pro · Four-system diagnostics
A handheld scanner aimed at engine, transmission, ABS, and airbag diagnostic work.
Chassis and ABS faults usually need a scanner that can read brake-system data directly.
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Universal · Torque wrench
Useful when reassembly torque matters for wheel, suspension, and under-hood fasteners.
Useful when wheel, hub, or suspension hardware needs correct reassembly torque.
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Universal · Drive-on ramps
Helpful for safer access underneath the vehicle during exhaust, EVAP, and underbody inspection work.
Helpful for safer access during underbody inspection work.
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