Chassis, generic

C05B8 Brake Booster Motor A Position Sensor Supply Circuit/Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

The rotor position sensor of booster motor A – Hall, magnetoresistive or a digital SENT angle sensor – is fed from a regulated 5 V output of the brake control unit. C05B8 reports that this feed is interrupted. No manufacturer documents the code publicly; GM's 2019 integrated brake control lists supply faults of its two motor position sensors under C05D1 and C05BB only. Denso's sensor patent shows why an open feed is hard to tell from a dead sensor: with a pull-down input the signal simply falls to 0 V. Hitachi Astemo's steering patent therefore monitors each sensor supply separately so the failed one can be identified. Without a rotor angle the booster cannot commutate its motor, so assist is cut back or lost and the brake warning appears.

Likely causes

Broken supply wire or pin between control unit and motor position sensor
Sensor supply output in the brake control unit failed
Internal open in the position sensor's supply path

Symptoms

  • Brake warning lamp and message such as service brake assist
  • Noticeably harder brake pedal
  • Regenerative or electronic brake functions unavailable

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,000
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

booster motor a pos sensor supplyelectric brake booster

Related codes

C05B9C05BAC05BBC05BCC0587

References: patents.google.com, patents.google.com, ti.com.