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DTC codes / C05xx · Brakes, steering and suspension / C05B8

C05B8Brake Booster Motor A Position Sensor Supply Circuit/Open

The C05B8 code indicates that the antilock brake system (ABS) control module has detected an open circuit or supply voltage problem in the wiring that feeds the position sensor for brake booster motor A. This sensor reports the motor's position to the control module, and without a valid signal the system cannot accurately manage brake assist. The module sets this code when the sensor's supply circuit is open or its voltage is outside the expected range.

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

Symptoms you may notice

  • Illumination of the ABS warning light and often the traction control or stability control warning light
  • A brake pedal that feels unusually hard or requires extra effort to push
  • Reduced or inconsistent power brake assist, especially during low-speed maneuvering
  • The brake system may default to a manual-assist mode with noticeably longer stopping distances
  • Often no symptoms besides the warning light, particularly if the open circuit is intermittent

Common causes, most likely first

  1. A broken, chafed, or corroded wire in the position sensor supply circuit, frequently at a connector or where the harness flexes
  2. A loose, backed-out, or corroded terminal at the brake booster motor position sensor connector
  3. A failed brake booster motor position sensor that internally opens the supply circuit
  4. A faulty ABS control module that fails to provide the reference voltage to the sensor
  5. Water intrusion or road salt corrosion in the connector or harness near the brake booster assembly
  6. A blown fuse or poor ground connection that interrupts power to the sensor circuit

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes and freeze-frame data, then clear the code and test-drive to see if it returns immediately or only under certain conditions
  2. Visually inspect the brake booster motor position sensor connector and the entire wiring harness for chafing, corrosion, or loose pins, and repair any damage found
  3. Use a digital multimeter to check for the specified reference voltage at the sensor connector with the key on, and check the ground circuit for continuity to chassis ground
  4. Measure the resistance of the sensor supply wire from the ABS module connector to the sensor connector to confirm an open circuit, and repair the wire if resistance is excessive
  5. If the wiring and connectors are good, test the position sensor with a multimeter or oscilloscope according to the manufacturer's procedure, and replace the sensor if it fails the test
  6. If the sensor and wiring check out, verify that the ABS control module is supplying the correct voltage and ground; replace the module only after all other possibilities are eliminated
  7. After repairs, clear the codes and perform a road test to confirm the warning lights stay off and the brake assist functions normally

Before you replace the brake booster motor

  • A common oversight is replacing the entire brake booster motor assembly when only the position sensor or its pigtail connector is faulty, so always test the sensor circuit first.
  • The code can be set by a momentary open circuit caused by vibration or temperature changes, so a wiggle test of the harness while monitoring live data often reveals the fault.
  • A weak or failing battery or charging system can cause low reference voltage that mimics a sensor supply circuit fault, so check system voltage under load before condemning parts.
  • Corrosion inside the connector that is not visible from the outside can create an open circuit, so unplug the connector and inspect the terminals closely for green or white deposits.
  • The ABS module may store this code along with other sensor codes if the shared reference voltage is lost, so diagnose the supply circuit before chasing multiple sensor faults.

Can I keep driving with C05B8?

Because this fault can reduce or disable power brake assist, the vehicle may require significantly more pedal effort to stop, so avoid driving until the repair is completed. If you must move the vehicle a short distance, do so at very low speed and be prepared for a hard brake pedal.

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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