Chassis, generic

C05BF Brake Booster Motor B Position Sensor Supply Circuit Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C05BF concerns a motor B sensor supply that stays inside its hard limits but behaves implausibly – it fluctuates, disagrees with the independent monitor, or does not match the first supply. No manufacturer documents C05BF publicly. GM's comparable check for its own two supplies, filed under C05BB and C05D1, shows the principle: brief excursions outside 4.85–5.15 V are weighted and accumulated, while good samples slowly reverse the count, so only a persistent disturbance latches. Hitachi Astemo chooses non-overlapping A/D windows of 475–575 and 699–849 counts for reference and monitor signals so that even a stuck comparator input is caught. A supply that drifts or ripples can corrupt the rotor angle without tripping simple low or high checks, so the module treats channel B as unreliable.

Likely causes

Intermittent contact or corrosion in the motor B sensor supply wiring
Unstable sensor supply regulator or failing filter capacitor in the control unit
Fault in the supply monitoring circuit itself

Symptoms

  • Brake warning lamp appearing at times
  • Assist occasionally reduced, code returns after clearing

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

C05BCC05BDC05BEC05BBC058E

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