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DTC codes / C05xx · Brakes, steering and suspension / C05B9

C05B9Brake Booster Motor A Position Sensor Supply Circuit Low

This code means the brake control module has detected an abnormally low voltage on the supply circuit that feeds the position sensor inside the brake booster motor assembly. The module expects a stable reference voltage on that sensor's power wire, and when the signal falls below the calibrated threshold, it stores C05B9 and typically disables the affected booster assist function.

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 and a message on the driver information display
  • A noticeably harder brake pedal with reduced power assist, especially during the first few presses after start-up
  • Intermittent or complete loss of the automatic brake booster function, requiring increased pedal effort to stop the vehicle
  • A stored code that returns immediately after clearing, sometimes with related booster motor or sensor codes present
  • Often no symptoms besides the warning light when the fault is intermittent or occurs only under specific voltage conditions

Common causes, most likely first

  1. A chafed, pinched, or corroded power wire between the brake control module and the brake booster motor position sensor
  2. A poor ground connection at the booster motor housing or the chassis ground point serving the brake control module
  3. A failing position sensor inside the brake booster motor assembly that draws excessive current and pulls the supply voltage down
  4. Low system voltage from a weak battery or a charging system that is not keeping up with electrical demand
  5. A faulty brake control module with an internal driver or reference voltage circuit that cannot supply the required current
  6. Corrosion or water intrusion at the booster motor electrical connector, particularly on vehicles where the booster is mounted low in the engine bay

How to diagnose and fix it

  1. Perform a battery and charging system test to rule out low system voltage before condemning any brake component
  2. Inspect the booster motor connector and the harness routing for chafing, corrosion, or pushed-out terminals, and repair as needed
  3. Measure the supply voltage and ground at the position sensor connector with the key on and compare to the manufacturer's specification
  4. Load-test the supply circuit with a test light or a current-limited jumper to confirm the wire can carry current without excessive voltage drop
  5. Check the ground circuit resistance from the booster motor housing to the battery negative terminal and clean or repair any high-resistance connection
  6. If the circuit tests good, substitute a known-good brake booster motor assembly or sensor and recheck for the code
  7. Clear the code and perform a road test with a scan tool monitoring the booster position sensor data to confirm the repair

What the supply circuit low code hides

  • A low supply voltage at the position sensor is often caused by a failing sensor that draws too much current, not by a bad wire, so replacing the sensor may be the correct fix even when the harness looks perfect.
  • Technicians frequently overlook the ground side of the circuit; a corroded ground strap on the booster motor can produce the same low-voltage code as a damaged power wire.
  • This code can set intermittently when the battery is weak or the charging system is marginal, and it may never return after the battery is replaced and the memory is cleared.
  • The brake control module may set this code as a consequence of a separate booster motor fault, so always check for related codes before replacing the module.
  • On some vehicles, the position sensor is not serviced separately from the booster motor assembly, which changes the repair strategy and cost significantly.

Can I keep driving with C05B9?

The vehicle remains controllable because the hydraulic brakes still work, but you will need noticeably more pedal effort and the booster assist may cut in and out unpredictably. Avoid heavy traffic and high-speed driving until the fault is repaired, and do not tow a trailer or carry a full load.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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

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