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DTC codes / C05xx · Brakes, steering and suspension / C05B0

C05B0Brake Hydraulic Circuit C Leak

C05B0 indicates that the anti-lock brake system (ABS) control module has detected a hydraulic fluid leak in circuit C, which is one of the isolated pressure circuits feeding a wheel brake. The module typically compares pressure sensor readings or pump motor behavior against expected values and flags the circuit when pressure decays faster than a calibrated leak threshold.

SystemChassis
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaBrakes, steering and suspension

Symptoms you may notice

  • Illumination of the ABS warning light, 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 a pulling sensation during hard braking
  • Visible brake fluid weeping or dampness around a wheel, caliper, or under the vehicle
  • The ABS may disable itself, reverting to conventional braking without anti-lock function
  • Often no symptoms besides the warning light when the leak is internal or very small

Common causes, most likely first

  1. A failing wheel cylinder, caliper piston seal, or brake line fitting allowing fluid to escape
  2. Corroded or cracked rigid brake line on the affected circuit
  3. An internal leak inside the ABS hydraulic control unit or pump
  4. A damaged or improperly seated brake hose crimp or banjo fitting
  5. A leaking pressure sensor or bleed screw on the affected circuit
  6. Master cylinder internal seal failure causing pressure loss on one circuit

How to diagnose and fix it

  1. Inspect all visible brake lines, hoses, calipers, and wheel cylinders on the affected circuit for wetness, corrosion, or damage
  2. Check the brake fluid reservoir level and look for evidence of external leakage under the vehicle
  3. Pressure-test the hydraulic circuit or perform a brake system leak-down test as specified by the manufacturer
  4. Repair or replace the leaking component, then bleed the affected circuit thoroughly to remove air
  5. Clear the diagnostic trouble code and verify the ABS warning light stays off during a road test
  6. If no external leak is found, test the ABS hydraulic control unit for internal leakage or faulty pressure sensors
  7. Confirm repair by monitoring live pressure sensor data and ensuring the code does not return after a complete drive cycle

What a C05B0 leak really means

  • An internal leak inside the ABS hydraulic unit can set this code without any visible fluid loss, so a dry driveway does not rule out the fault.
  • A sticking caliper slide pin or collapsed brake hose can mimic a hydraulic leak by causing uneven pressure decay that the module interprets as a leak.
  • The code identifies circuit C, but on many vehicles that circuit feeds a specific wheel; misreading which wheel is affected leads to replacing the wrong caliper.
  • Air trapped in the hydraulic system after a recent brake service can trigger a leak code because the compressible air changes pressure decay rates.
  • Some vehicles set a companion code for the pump motor or pressure sensor, and chasing only the leak code without addressing the sensor can leave the light on.

Can I keep driving with C05B0?

Do not continue driving with a confirmed hydraulic leak, because brake performance can deteriorate suddenly and ABS may be disabled. Have the vehicle towed or driven only to the nearest repair facility at low speed if the pedal still feels firm and fluid level is stable.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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

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