New Purchase of Used Auto Repair Issues?

Long story short I bought a used lancer 2wks ago no warranty. The "service engine soon'' light goes on and cont. into the next day. I call the dealership where I bought the car- 1st they wanted to charge me just to figure out the problem 2nd- once they found out it was the cat. converter they will charge me around $750 to fix it. They are refusing the do it for free. I could understand if I had it for a couple of month but, come on its been 2 wks. What should I do???