Chassis, generic

C05BD Brake Booster Motor B Position Sensor Supply Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C05BD means the supply of motor B's position sensor reads below its permitted minimum – a ground short, a sensor pulling excess current or a weak regulator. No manufacturer publishes this code. Hitachi Astemo explains why two supplies need cross-checking: both share one ground, so without comparison the controller could not say which supply sagged. Each microcontroller there reads a 2.5 V reference from its own supply and a 3.75 V monitor of the other, and a low supply leaves a distinct pattern across those four values. A tracking regulator like the TPS7B4253-Q1 from TI holds its output current at a limit during a short, so the voltage drops instead of the part burning out. With channel B lost, the actuator relies on motor A or its first sensor and may lower assist gradually.

Likely causes

Motor B sensor supply line shorted to ground
Damaged motor B position sensor drawing too much current
Second sensor regulator in the control unit weak

Symptoms

  • Brake warning lamp on
  • Assist fades gradually or is limited at high demand

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 b pos sensor supplyelectric brake booster

Related codes

C05BCC05BEC05BFC05B9C058C

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