C055DBrake Hydraulic Circuit B Leak
- Generic OBD-II code · Chassis
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- Passenger-car brakes are split into two hydraulic circuits so that one can still stop the car if the other fails. Brake-by-wire units with a motor-driven plunger can estimate fluid loss directly: they compare the plunger travel with the measured pressure. In GM's 2025 EBCM tables C055D is the leak check of one circuit (listed as circuit 1): if the calculated leakage passes 500 mm³/s (a second stage 2000 mm³/s) for 0.1–0.5 s while braking with the circuits separated, a Type A fault with MIL is set after one trip. C055E covers the other circuit. FMVSS 135 requires a red brake warning when a split system suffers a major drop in fluid level or pressure. The car then brakes with reduced effect and longer pedal travel; it must be checked before further driving.
- What you may notice
- Red brake warning and amber ABS or stability lamp
- Soft or sinking brake pedal, longer stopping distance
- Falling fluid level or wet spots at a wheel
- Likely causes
- LikelyLeaking brake line, hose, fitting or caliper in this circuit
- PossibleWorn seal in the master cylinder or the brake-by-wire unit
- PossibleAir left in the circuit after service, read as compliance or leakage
- RareOverly sensitive leak calculation in the brake module software
- Repair
- Hard · Usually a job for a workshop.Typical cost: €60 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.