Chassis, generic

C05CB Brake Master Cylinder Piston Position Sensor A Circuit Voltage Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Mirror image of C05CA: channel A of the master cylinder piston travel sensor reports that its value fell below the permitted range. GM documents it for its booster-type EBCMs, where the linear position sensor encodes travel digitally and flags its own low limit violation: the XT4 module acts after 0.033 s on the SENT fast channel, while the 2023 and 2025 tables wait 0.96 s for the slow-channel status, Type A, one trip, MIL. A low report often follows a sensor that loses its magnet reference or sees a sagging supply. Bosch builds such travel sensors around a Hall element that tracks a magnetic field over up to 45 mm of rod stroke. If the code returns after wiring checks, the sensor is usually part of the booster unit and replaced with it.

Likely causes

Position sensor A reporting out-of-range low
Sagging sensor supply voltage
Magnet carrier loose or damaged

Symptoms

  • Malfunction indicator lamp and brake system message
  • Brake assist reduced, regeneration possibly off

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,000
Time0.5–3 h

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

Parts involved

master cylinder pos sensor aelectric brake booster

Related codes

C05CAC05CCC05CEC05D1

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, eenewseurope.com.