Legal Question in Business Law in California

New or used? Why did it "break down?" That's a pretty broad phrase.

A new car would be covered by warranty of some kind; you'd have to read it and consult with the dealer about warranty repairs, but you can't just return it. After awhile, if it turns out to be unrepairable, the car might fall under the so-called "lemon laws", see Civil Code section 1793.22, and the dealer might have to take it back.

With a used car, the possibilities are broader, depending upon whether you bought from a dealer or a private party, and how the car was described and what your contract says. Some used cars are sold with express and/or implied warranties, and some are sold "as is." It then depends upon the terms of the deal, and perhaps to some extent, exactly what went wrong. In any case, the seller probably has an option to make repairs rather than take back the vehicle and refund your money.