Chassis, generic

C05B0 Brake Hydraulic Circuit C Leak

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C05B0 reports lost fluid or volume in a hydraulic circuit the maker designates C; at GM its meaning has changed over time. GM's 2016–2019 hybrid brake controllers used it for the front axle circuit: wheel behaviour not matching vehicle deceleration during ABS or EBD for 1 s, or master cylinder pressure above rear axle pressure at constant pedal. A 2019 EBCM with integrated booster flags dynamic leaks when a pressure error integral passes 20 bar. In GM's 2023 and 2025 tables C05B0 means air in the plunger circuit: a volume deviation over 2 cm³ during replenishment, or over 1.5 cm³ in the other tests. All Type A with MIL. GM bulletin 22-NA-247 warns of false C05B0 on eBoost models after pad work; pump the pedal firm before clearing. On 2019 Silverado/Sierra it came with Service Brake Assist.

Likely causes

Caliper pistons pushed back during pad work and not reseated before driving (GM 22-NA-247)
Air in the plunger or brake circuit after service or bleeding
Real fluid leak at a line, hose, caliper or valve
Brake module software calibration causing false detection (GM PIT5638)

Symptoms

  • Service brake assist or brake system message, brake warning lamp
  • Longer or softer pedal travel
  • MIL on (GM)

Repair estimate

DifficultyHardWorkshop job
Parts and labour€50–1,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake circuit cabs hydraulic unit

Related codes

C05B1C055DC055EC055FC058EC0049

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.