Chassis, generic

C05CD Brake Master Cylinder Piston Position Sensor B Circuit Voltage High

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Channel B of the master cylinder piston travel sensor is stuck at a permanent high level. In GM's booster-type EBCMs the second channel is not sent over SENT like channel A but as a PWM signal around 1 kHz, which is why high and low here mean a line that no longer toggles: the module sets C05CD when it sees a steady high state for 0.2 s, read as a short of the PWM line to supply (19OBDG03D XT4 section, 23OBDG07, 25OBDG03B; Type A, one trip, MIL). Duty cycle or frequency outside 8.5-92 % or 900-1120 Hz falls under a separate communication code. Look for chafed wiring touching a supply wire or a sensor output stage that has failed high. C05CE is the stuck-low partner.

Likely causes

PWM signal line shorted to supply voltage
Output stage of sensor channel B failed high
Moisture or corrosion bridging pins in the connector

Symptoms

  • Malfunction indicator lamp and brake warning on GM models
  • Booster runs on channel A only or reduces assist

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–2,000
Time0.5–3 h

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

Parts involved

master cylinder pos sensor bsensor wiring

Related codes

C05CEC05CFC05CA

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