External brake fluid leak from a rusted or damaged brake line in circuit B, most commonly at a rear wheel or along the underbody; Leaking wheel cylinder or caliper seal on the circuit B axle, allowing fluid to escape past the piston; Internal leak inside the ABS hydraulic control unit, such as a worn solenoid valve or pump seal that lets pressure bleed off internally; Failed pressure sensor or faulty wiring to the sensor that reports incorrect pressure for circuit B, triggering a false leak detection; Master cylinder primary or secondary piston seal failure that affects the circuit B pressure circuit; Air trapped in circuit B after recent brake service, causing pressure readings that mimic a leak; Corroded or damaged ABS module connector pins that interrupt the pressure signal for circuit B

DTC codes / C05xx · Brakes, steering and suspension / C055D

C055DBrake Hydraulic Circuit B Leak

Q: What does code C055D mean?

Brake Hydraulic Circuit B Leak. C055D indicates that the antilock braking system (ABS) module has detected a hydraulic pressure loss or fluid leak in the brake circuit designated as 'B', which is typically the rear hydraulic channel on most vehicles. The module monitors pressure sensor readings and pump motor activity, and it sets this code when the pressure in circuit B cannot be maintained or decays faster than expected during a self-test or normal braking.

C055D indicates that the antilock braking system (ABS) module has detected a hydraulic pressure loss or fluid leak in the brake circuit designated as 'B', which is typically the rear hydraulic channel on most vehicles. The module monitors pressure sensor readings and pump motor activity, and it sets this code when the pressure in circuit B cannot be maintained or decays faster than expected during a self-test or normal braking.

SystemChassis

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaBrakes, steering and suspension

Symptoms you may notice

ABS warning light illuminated on the dashboard, often accompanied by the traction control and stability control lights
A soft or spongy brake pedal that may sink slowly when held under steady pressure
Longer stopping distances or reduced brake assist during emergency braking
Visible brake fluid weeping or pooling near a wheel, caliper, or along the brake lines
The ABS pump motor may run longer than normal or cycle repeatedly after the engine is shut off
Often no symptoms besides the warning light, especially if the leak is small and internal to the hydraulic unit

Common causes, most likely first

External brake fluid leak from a rusted or damaged brake line in circuit B, most commonly at a rear wheel or along the underbody
Leaking wheel cylinder or caliper seal on the circuit B axle, allowing fluid to escape past the piston
Internal leak inside the ABS hydraulic control unit, such as a worn solenoid valve or pump seal that lets pressure bleed off internally
Failed pressure sensor or faulty wiring to the sensor that reports incorrect pressure for circuit B, triggering a false leak detection
Master cylinder primary or secondary piston seal failure that affects the circuit B pressure circuit
Air trapped in circuit B after recent brake service, causing pressure readings that mimic a leak
Corroded or damaged ABS module connector pins that interrupt the pressure signal for circuit B

How to diagnose and fix it

Inspect the entire circuit B hydraulic path for external leaks, starting at the rear wheels and working forward along the brake lines to the ABS unit
Check brake fluid level in the master cylinder reservoir and note whether it has dropped since the last service
Use a scan tool to read live ABS pressure sensor data for circuit B and compare it to expected values during a commanded pump test
Pressure-test circuit B with a hydraulic gauge or perform a controlled hold test to confirm whether pressure decays and at what rate
Inspect the ABS module connector and pressure sensor wiring for corrosion, chafing, or loose pins before condemning the hydraulic unit
Repair or replace the leaking component, then bleed circuit B thoroughly and clear the code to verify the fault does not return
If no external leak is found and the pressure sensor and wiring test good, replace or rebuild the ABS hydraulic control unit and perform the required calibration

What a circuit B leak really means

Circuit B is not always the rear brakes on every make; some manufacturers assign the letters differently, so confirm the circuit layout in the service information before chasing a leak at the wrong wheel.
An internal leak inside the ABS hydraulic unit can set C055D with no visible fluid loss anywhere on the vehicle, which is why pressure testing and live sensor data matter more than a visual inspection alone.
A faulty pressure sensor or a corroded connector can report a pressure drop that does not actually exist, so technicians who replace the ABS unit without checking the sensor signal first often misdiagnose this code.
Air in circuit B after a recent brake job or a poorly executed bleed can cause pressure readings that the module interprets as a leak, so always confirm the system is properly bled before replacing parts.
This code can be stored alongside wheel speed sensor codes or pump motor codes, and those companion codes sometimes point to the real root cause rather than a hydraulic leak.

Can I keep driving with C055D?

Do not continue driving normally with C055D because a hydraulic leak in circuit B can reduce braking performance and may worsen suddenly; have the vehicle inspected and repaired before driving any distance beyond a safe pull-off, and avoid heavy braking or towing.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.