C05B1Brake Hydraulic Circuit D Leak
- Generic OBD-II code · Chassis
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- C05B1 is the leak code for hydraulic circuit D. The only public maker use is GM's hybrid brake controller of 2016–2019, where it covers the rear axle circuit. Two checks feed it: during pressure hold phases with the pump off and the car at rest, a calculated rear leakage above 0.3 ml/s, judged over more than 300 s of cumulative braking; and, with more than 5,000 kPa at the front axle and a steady pedal for 3 s, a master cylinder pressure lower than the rear axle pressure. Type A, MIL after one trip. Its partner C05B0 covered the front axle there. A Continental patent shows another approach: build pressure with a motor-driven plunger and judge tightness from the slope of the pressure decay. FMVSS 135 demands a red warning on major fluid loss. Inspect lines, rear calipers and the hydraulic unit.
- What you may notice
- Brake and ABS warning lamps on, MIL possible
- Longer pedal travel and weaker rear braking
- Likely causes
- LikelyFluid leak at a rear brake line, hose or caliper
- PossibleInternal leakage of an inlet or outlet valve in the hydraulic unit
- PossibleAir in the rear circuit after service
- Repair
- Hard · Usually a job for a workshop.Typical cost: €50 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.