Chassis, generic

C0021 Brake Booster Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Booster worn or not holding vacuum/pressure
Low battery voltage during module programming
Faulty boost or pressure sensor input

Symptoms

  • Hard brake pedal and reduced assist
  • Service brake or reduced power message
  • ABS or brake warning lamp on

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€250–1,800
Time1–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake booster

Related codes

C0022C0047C0048

References: static.nhtsa.gov, en.wikipedia.org.