C0021: owner reports and bulletins

Chassis. We haven't written the explanation for this code yet. Below is what owners and manufacturers have filed with NHTSA.

Got C0021 on your scanner? Tell the mechanic your year, make, model and any symptoms and you'll get the likely cause and what to check first. Ask about this code.

What owners report to NHTSA

13 complaints filed with NHTSA mention C0021, 13 of them since 2015, across 9 different models. The component owners most often filed it under: service brakes.

Most-reported models

VehicleComplaintsModel years reported
CHEVROLET SILVERADO 150042019

Manufacturer service bulletins that name C0021

18 technical service bulletins and manufacturer communications filed with NHTSA mention C0021, from 4 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:

  • MAZDA MAZDA3 (2019, 2020, 2021), bulletin 10199722: When switching to the maintenance mode, U3000:53 and the related DTCs are stored, but the DTCs will not be cleared automatically after finishing the maintenance mode. It is necessary to clear the DTCs completely after pe…
  • CHEVROLET SILVERADO 1500 (2020), bulletin 10171256: This Preliminary information communicates repair information onService Brake System, Reduced Engine Power, and Speed Limited messageson the DIC. Along with these messages, a SES MIL will be on and the brake pedal may fee…
  • GMC SIERRA 1500 (2020), bulletin 10171256: This Preliminary information communicates repair information onService Brake System, Reduced Engine Power, and Speed Limited messageson the DIC. Along with these messages, a SES MIL will be on and the brake pedal may fee…
  • CADILLAC XT6 (2020, 2019), bulletin 10181688: This technical bulletin provides repair information to correct a customer concern of Loss of Brake Assist, Various Messages Displayed in Driver Information Center (DIC), Brake Pedal Travel and Apply May Feel Different, O…

Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.