Legal Question in Business Law in California

If there was a written agreement to service the car, they have four years. If it was verbal, they have two.

Yes. You asked them to perform the repairs and you got the benefit of their work, so you have to pay for their services. At some point, though, the limitations period on the mechanics' claim against you will expire. If you still haven't paid by that point and they haven't sued you for the money, you will no longer have to pay.

Yes as long as don't wait too long so they can sue you if you don't pay. See Tim's answer above as to deadlines. Also it is four years if it is an account stated or open book account.