Brother HL-9420 Toner
Many consumers are worried if the use of compatible or aftermarket printing supplies will void their printer's warranty. The answer is no.
More simply put, this means that your printer warranty cannot be voided just because you choose to use compatible products unless the manufacturer can prove that the compatible product caused direct damage to your printer. In cases such as this, the manufacturer may choose not to repair your printer but your warranty would remain in effect for all other warranty issues.
For more information see:
United States Code Annotated
Chapter 50 Consumer Product Warranties 15 Section 2302 Section C