what does your written contract or estimate say about the scope of work, the time it will take, and any warranty the mechanic is providing? You're essentially asking a bunch of strangers "what does my contract say" and we have no idea. If you had no written contract or estimate and just paid in advance by check, you really need a lawyer for help, because you left yourself with no protection at all.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.