If the mechanic did not perform the repairs he said he would, or performed them negligently (unreasonably carelessly), you should not have to pay for them. You have two ways to proceed:1) Wait to see if the mechanic takes legal action against you, then defend against his claim or suit; to win, you'd have to show that he did not do the work or was negligent, which may require bringing in a mechanic of your own to testify.2) Affirmatively bring a suit of your own seeking a "declaratory judgment," or court determination as to how much you owe. Again, you'd need evidence. This option puts the burden on you, but lets you get certainty.Given the cost and time of litigation, and that the outcome is never certain, your best bet may be to settle (if you can) for some mutually agreeable amount.