01 How to Avoid Engine Damage from Engine Flush • 02 6-Speed DSG Automatic Transmission Common Faults: Causes, S… • 03 Why Aren’t the Rear Tires on Cars Made to Turn Like the Fro… • 04 This is How A Slipping Transmission First Reveals Itself Lo… • 05 This Is Why You’ll Want an Electric Car by 2030 • 06 Slash Fuel Consumption by Almost Half: The One Thing You Ne… • 07 Why Your Check Engine Light Is On: 15 Common Causes and DIY… • 08 Toyota Invests in New Battery Technology, Eyes 1.5 Million … • 09 P0101 - Mass Air Flow (MAF) Sensor Performance • 10 How to Calibrate Your Car’s Sensors After Repairs (ABS, TPM… •

DTC codes / C02xx · Brakes, steering and suspension / C0266

C0266Active Brake Booster

C0266 indicates that the anti-lock brake system (ABS) control module has detected a fault in the active brake booster circuit, which is part of the vehicle's stability and brake assist system. The computer monitors the booster's electrical circuit and pressure performance, and it sets this code when the booster fails to respond as commanded or the circuit is out of range.

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

Symptoms you may notice

  • Illumination of the ABS warning light, and often the traction control and stability control lights as well
  • A hard brake pedal that requires noticeably more effort to stop the vehicle
  • Reduced or absent brake assist, making emergency stops feel less powerful
  • The stability control system may be disabled, and the vehicle may handle differently in slippery conditions
  • Often no symptoms besides the warning light, especially if the fault is intermittent or only affects the booster's diagnostic circuit

Common causes, most likely first

  1. Failed active brake booster assembly, including internal solenoid or pressure sensor faults
  2. Damaged or corroded wiring and connectors at the booster or ABS module
  3. Blown fuse or poor ground connection for the booster circuit
  4. Faulty brake pedal travel sensor or booster position sensor
  5. Low brake fluid level or air in the hydraulic system affecting booster performance
  6. ABS control module internal fault or software issue
  7. Vacuum supply problem on vacuum-assisted active boosters, such as a cracked hose or failed check valve

How to diagnose and fix it

  1. Check for related ABS and stability control codes and record all freeze-frame data before clearing anything
  2. Inspect the brake fluid level and condition, and verify the booster's electrical connectors and wiring for corrosion or damage
  3. Test the booster circuit with a digital multimeter for power, ground, and signal voltage according to the manufacturer's specifications
  4. Perform a bidirectional scan-tool test of the active brake booster to command it on and off and observe pressure changes
  5. Check vacuum supply and the booster check valve if the system is vacuum-assisted
  6. If electrical and hydraulic checks pass, replace the active brake booster assembly and perform any required calibration or bleeding procedures
  7. Clear codes, road-test the vehicle, and verify that the warning lights stay off and brake assist functions correctly

What the active brake booster code really means

  • A low brake fluid level or a recent brake job that introduced air into the hydraulic system can trigger C0266 even when the booster itself is healthy, so always verify fluid level and bleed the system before condemning the booster.
  • The active brake booster is often integrated with the master cylinder and ABS module, and a failure in any of those components can set this code, so technicians should not assume the booster alone is at fault.
  • Intermittent C0266 codes are frequently caused by a chafed wire harness near the booster or a poor ground connection that only acts up under vibration or temperature changes.
  • Some vehicles require a scan-tool calibration or a relearn procedure after booster replacement, and skipping that step will leave the code stored even with a new part installed.
  • A failing brake pedal travel sensor can mimic a booster fault, so check live data for pedal position and booster command before replacing expensive hardware.

Can I keep driving with C0266?

Because this fault can reduce brake assist and disable stability control, you should avoid driving the vehicle until it is diagnosed, especially at highway speeds or in wet conditions. If you must move it, drive slowly and leave extra stopping distance, and do not tow a trailer or carry heavy loads.

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.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage