There are plenty of possibilities and not enough information here to say how to proceed. Most use car sales are sold on an "as is" basis. You were not clear as to whether you have a warranty, a service contract OR whether any of the work was being done by the dealer OR if there are writings to evidence the work. Once we sift through all of that it's possible you will have some recourse as this appears to be a breach of the "implied warranty of merchantability" which may allow you to unwind the deal.
There are other possible warranties that may have been created at sale, for instance if you stated you needed a particular kind of vehicle to do a particular kind of job and they said this vehicle could get the job done. There a warranty of fitness would be created and apparently breached.
Find a good consumer lawyer in AZ, Hyung Choi comes to mind and see what kind of help you may be able to get.
OR search http://www.naca.net for someone to help.
Good luck with it.