C05D2Master Cylinder Piston Position Higher Than Expected
- Generic OBD-II code · Chassis
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyBrake module software issue (GM 20-NA-073, reprogramming)
- PossibleInternal leakage at master cylinder or simulator seals
- PossibleSimulator pressure sensor reading too low within range
- RareAir in the hydraulic system after service
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.