C055D code meaning
Brake Hydraulic Circuit B Leak
Chassis system Generic code C05xx codes Check engine light
What C055D means
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.
C055D at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C055D
- 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
Common causes of C055D
- Most 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
- Less likelyOverly sensitive leak calculation in the brake module software
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake circuit bABS hydraulic unit
What each part of C055D means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5D | The exact fault in that area. |
What to do next
- Note C055D and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C055D
What does C055D mean?
C055D means "Brake Hydraulic Circuit B Leak". Passenger-car brakes are split into two hydraulic circuits so that one can still stop the car if the other fails. It is a generic code, so it means the same on every make.
Is it safe to drive with C055D?
Drive gently and only to a workshop. With C055D the car can go into limp mode (reduced power) to protect itself.
Will C055D fail an emissions test?
C055D is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C055D to fix?
Difficulty: hard. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will C055D clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.