Keep me logged in until I sign out. You may not reject a car already accepted, unless you accepted it based on the assumption that the seller would repair the defect within a reasonable period. Your option now is to revoke your acceptance. You must give the seller notice of the defect, and show that it substantially impairs the value of the car to you.
Revocation involves a higher standard than rejection, and different states have various standards.
Generally, the defect will have to be major to allow revocation. After revoking acceptance, you must act as if you had originally rejected the car. Leave the car in your driveway until the seller reclaims it, or return it yourself. Everything you need for court related issues. The Intelligent Alternative to Court.