C0021: Brake Booster Performance
The brake booster is not providing the expected assistance.
High risk
Have it checked soon. The brake pedal may feel hard and stopping distances may increase.
Short answer · Can I keep driving with C0021?
Only with great care. The brake pedal may feel hard and stopping distances may increase. Have it checked soon.
What does C0021 mean?
C0021 points to the brake booster, which multiplies the force of your foot. A vacuum leak, a failed vacuum pump on diesel or turbo engines, or a faulty electric booster are typical causes.
What the car does when the code is set
The brake warning light may come on, and brake assistance or ABS functions may be limited.
How the system works
Modern brakes use sensors for fluid level, pedal position, pressure and pad wear. The brake control unit uses them for ABS, brake assistance and warnings.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Low brake fluid levelWorn pads or a leak lower the level in the reservoir.
- Faulty sensorPressure, travel or level sensors can fail internally.
- Wiring or connector faultDamaged wires or corroded plugs give wrong signals.
- Air or moisture in the brake fluidOld fluid or a leak changes how the system behaves.
- Brake booster or hydraulic unit faultPossible once fluid, sensors and wiring have been checked.
How it is diagnosed
- Check the brake fluid levelWith the car level, the fluid should sit between MIN and MAX.If the level is low or the pedal feels soft, do not drive; have the car recovered.
- Look for leaksCheck behind each wheel and around the master cylinder for fluid.
- Check the padsWorn pads lower the fluid level and may trigger wear warnings.
- Read live dataA workshop compares sensor values with the pedal at rest and pressed.
Questions about C0021
Why is the brake pedal hard?
Without booster assistance you need much more force on the pedal to stop.
Is a brake system code an emergency?
It can be. If the brake warning light is red or the pedal feels different, stop driving and have the car checked.