Chassis, generic

C05B9 Brake Booster Motor A Position Sensor Supply Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C05B9 is set when the feed voltage for motor A's position sensor sags below its lower limit. No manufacturer publishes C05B9; as a reference point, GM's 2019 brake module calls its 5 V sensor supplies low below 4.85 V lasting 100 ms, but books that under C05D1 or C05BB. A low supply usually means the line is shorted to ground or a sensor is drawing too much current. Supplies of this kind are often tracking regulators such as TI's TPS7B4253-Q1, built for sensors on long cables: the output is short-proof to ground and current-limited at typically 450 mA, so instead of failing it lets the voltage collapse. Since the same rail can also feed pedal sensors, related codes may appear together. Motor commutation becomes unreliable and assist is reduced or removed.

Likely causes

Supply line chafed through and shorted to ground
Defective position sensor or another sensor on the same rail loading the supply
Weak sensor supply regulator in the control unit

Symptoms

  • Brake warning lamp, often with several sensor codes at once
  • Brake pedal feels hard, assist reduced

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

C05B8C05BAC05BBC05BDC05D1

References: ti.com, gsi.ext.gm.com, patents.google.com.