Chassis, generic

C05D5 Brake Master Cylinder Cut Off Valve

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Electrical fault of the solenoid valve that separates the master cylinder from the wheel circuits in by-wire operation, so pressure comes from the motor and the pedal works against a simulator. In GM's booster-type EBCMs (XT4, 23OBDG07, 25OBDG03B) the valve driver reports over-current of about 4 to 6.5 A, an output stage above 195-220 °C, a lost power ground or lost freewheeling within 0.03 s. GM's integrated controller maps open and shorted driver tests of its simulator-side valve to C05D5 (Type A, MIL) but a coil above 265 °C as Type C without MIL; 25OBDG04A uses it for the traction control isolation valves. Bosch describes these isolation valves closing during by-wire braking. Expect reduced assist or a mechanical push-through fallback.

Likely causes

Valve coil shorted or open inside the hydraulic unit
Valve driver stage or ASIC in the module defective
Poor power ground of the brake module
Valve overheated by excessive activation time

Symptoms

  • Malfunction indicator lamp and brake warning
  • Loss of brake assist, pedal works on the master cylinder directly
  • ESC and traction control limited

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€300–3,000
Time1–4 h

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

Parts involved

master cylinder cutoff valvebrake system control module

Related codes

C05D2C05D3C0595

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