C0021 code meaning
Brake Booster Performance
What C0021 means
A performance code for the power brake booster, set when the booster does not deliver the expected level of assist or does not respond to the control module as commanded. On vehicles with an electronic or vacuum booster the module compares driver pedal demand, measured boost and the resulting brake pressure, and stores the code when assist is weak, slow or inconsistent. GM documents C0021 setting on 2019-2020 Silverado and Sierra together with reduced engine power and service brake messages, in some cases after a module programming event with low battery voltage. The driver may feel a hard pedal and longer stopping distance; a warning message and the brake or ABS lamp appear.
C0021 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor1 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C0021
- Hard brake pedal and reduced assist
- Service brake or reduced power message
- ABS or brake warning lamp on
Common causes of C0021
- Most likelyBooster worn or not holding vacuum/pressure
- PossibleLow battery voltage during module programming
- Less likelyFaulty boost or pressure sensor input
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Brake booster
What each part of C0021 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 21 | The exact fault in that area. |
What to do next
- Note C0021 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C0021
What does C0021 mean?
C0021 means "Brake Booster Performance". A performance code for the power brake booster, set when the booster does not deliver the expected level of assist or does not respond to the control module as commanded. It is a generic code, so it means the same on every make.
Is it safe to drive with C0021?
Drive gently and only to a workshop. With C0021 the car can go into limp mode (reduced power) to protect itself.
Will C0021 fail an emissions test?
C0021 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C0021 to fix?
Difficulty: workshop only. A typical repair takes 1 to 4 hours of work. This is a job for a workshop.
Will C0021 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.