C0266
Active Brake Booster
The brake control module has detected a fault in the active (electric or vacuum-controlled) brake booster. Active boosters are used by emergency brake assist and adaptive cruise control to autonomously build brake pressure.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: hard
Symptoms
- ABS warning light on
- ESC/stability control deactivated
- Possible reduced brake assist
- Speedometer may behave erratically
Common causes
Ordered by how often each turns out to be the cause.
- Active booster motor or solenoid failedCommon
- Vacuum supply weak (vacuum pump or hose fault)Possible
- Wiring or connector faultLess likely
Parts involved
- Active Brake Booster
- Brake Control Module
- Vacuum Pump
C0266 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.