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DTC codes / C05xx · Brakes, steering and suspension / C05BA

C05BABrake Booster Motor A Position Sensor Supply Circuit High

The anti-lock brake system (ABS) module has detected that the voltage on the supply circuit for the brake booster motor position sensor is higher than the expected range, typically near battery voltage when it should be a regulated lower reference. This high supply voltage can prevent the sensor from reporting the booster motor's true position, so the ABS module sets code C05BA and may disable the brake booster assist.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaBrakes, steering and suspension

Symptoms you may notice

  • The ABS warning light and often the red brake warning light illuminate on the dashboard.
  • The brake pedal may feel unusually firm or require extra effort because the brake booster motor assist is disabled.
  • A message such as 'Brake System Fault' or 'Service Brake System' may appear in the driver information center.
  • The brake booster motor may run continuously or not at all, depending on how the module reacts to the faulty supply.
  • Often no symptoms besides the warning light, with normal brake feel in many cases.

Common causes, most likely first

  1. A short to voltage on the sensor supply wire between the ABS module and the brake booster motor position sensor.
  2. A faulty brake booster motor position sensor internally shorted to its supply voltage.
  3. Corrosion or water intrusion in the connector at the brake booster motor position sensor or the ABS module.
  4. A damaged wire harness where the supply circuit rubs against a metal bracket or passes through a tight grommet.
  5. A failed ABS module with an internal short on the sensor supply output.
  6. A poor ground connection for the brake booster motor or ABS module that causes the supply voltage to float high.

How to diagnose and fix it

  1. Connect a scan tool and record all ABS and brake system codes, then clear codes and verify whether C05BA returns immediately.
  2. Inspect the brake booster motor position sensor connector and the ABS module connector for corrosion, water, or loose terminals.
  3. Back-probe the sensor supply circuit at the sensor connector with the key on and measure voltage; compare it to the manufacturer's specification.
  4. Disconnect the sensor and the ABS module, then check the supply wire for continuity to battery voltage with both ends isolated to rule out a short to voltage.
  5. If the supply voltage is correct at the sensor connector, test the position sensor with a known-good unit or follow the manufacturer's sensor sweep test.
  6. If the supply circuit and sensor test good, substitute a known-good ABS module or perform module reprogramming as directed by the manufacturer.
  7. After repair, clear codes, perform any required ABS calibration or booster motor learn procedure, and verify the warning lights stay off during a road test.

What a high supply voltage really means

  • A high supply voltage code does not automatically condemn the sensor; a short to voltage on the supply wire or a bad ground can produce the same reading.
  • Water intrusion into the brake booster motor connector is a common hidden cause, especially on vehicles where the booster sits low in the engine bay.
  • The ABS module may set this code intermittently when the booster motor runs and draws current, so a static voltage check with the motor off can miss the fault.
  • A weak or corroded ground for the ABS module can make the sensor supply reference float high, mimicking a short to voltage.
  • Replacing the brake booster motor position sensor without checking the supply circuit often leads to a repeat failure and a second repair bill.

Can I keep driving with C05BA?

You can usually drive the vehicle to a repair shop, but expect a firm brake pedal and reduced or absent brake booster assist, so increase following distance and avoid heavy traffic until the fault is repaired.

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.

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