What can I do if the car I recently bought had a problem and previous owner didn't fix it before selling?

I purchased a 2005 Kia , "as is contract" from a dealer in Memphis, TN. Within two weeks of using the vehicle the engine light indicator came on and would not go off. I contacted my personal mechanic who says the engine was beyond repair and need to be replace. I took the car back to the dealer and they agreed to use there mechanic to access the problem. It has been several weeks now since they took the vehicle and the problem still hasn't been fix. It turn out it was a problem that was present before I bought the car. It was not stated in the car fax. I was told I had to pay to fix the vehicle. Can you advice me of what my rights are?