C0021: Brake Booster Performance
What to do
The pedal may become much harder to press, and you may need to push far harder than usual to stop. The brakes still work, but plan for longer stopping distances and much more effort. Get this looked at promptly — if the pedal is already hard, drive gently and slowly, or have the car recovered.
What this code means
The brake booster is not performing as expected.
The booster is what makes the brake pedal light, and losing it is the one fault in this range a driver will feel immediately.
A brake system needs far more force than a leg comfortably provides. The booster multiplies pedal effort — traditionally using engine vacuum against a diaphragm, and increasingly using an electric pump or motor, because a hybrid whose engine stops and an EV with no engine at all have no vacuum to borrow.
A performance code means the booster responded but did not achieve the assistance asked of it. So this is not usually wiring: it is a vacuum leak, a failed check valve, a tired vacuum pump, or a booster that has lost its seal.
What the driver notices is a hard pedal. The brakes still work — you can stop the car — but it takes a great deal more effort, and the stopping distance goes up simply because most people do not push hard enough. That is worth knowing before it happens rather than discovering it at a junction.
A quick check anyone can do: with the engine off, press the brake pedal several times to use up the stored vacuum, hold it down, and start the engine. The pedal should sink slightly as assistance arrives. If it does not, the booster is not getting vacuum.
Likely causes
Start here
- Leaking brake servo or servo hose · specialist
- Faulty brake booster check valve · moderate
- Leaking brake booster diaphragm · specialist
Less common
- Faulty vacuum pump · specialist
Rare
- Faulty stability control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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