Chassis, generic

C05D2 Master Cylinder Piston Position Higher Than Expected

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

The piston has travelled further than the measured pressure justifies, a sign that fluid escapes or a sensor misreads. GM's integrated controller models the simulator pressure from pedal travel and sets C05D2 when the model exceeds the measured value by more than 20 bar (250 ms in 19OBDG03D, 500 ms in 25OBDG04B), naming master cylinder or simulator seal leakage and an in-range pressure sensor fault. The XT4 module reacts after 0.2 s when pressure lags behind the measured travel. GM bulletin 20-NA-073 (2019-2020 pickups and SUVs) lists C05D2 as excessive piston travel with loss of brake assist and fixes it by reprogramming the BSCM. Bosch and Continental patents describe the same travel-versus-pressure leak check. C05D3 is the opposite case.

Likely causes

Brake module software issue (GM 20-NA-073, reprogramming)
Internal leakage at master cylinder or simulator seals
Simulator pressure sensor reading too low within range
Air in the hydraulic system after service

Symptoms

  • Longer, softer brake pedal with higher pedal force
  • Loss of brake assist and messages in the driver information centre
  • Malfunction indicator lamp on GM models

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,500
Time0.5–4 h

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

Parts involved

master cylinderpedal simulatorbrake system control module

Related codes

C05D3C05D5C053D

References: gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov, patents.google.com.