Got a check engine like on early this week. I took it to midas because I needed to get my fuel filter changed and injectors clean. They did the reading for me and said it was a bad CAT. I was quoted $400 to have it replaces. I dont' know a whole lot about CATs. The only thing I did to my car was put a magnaflow exhaust on it long time ago. Do I need to replace the whole system or part of it? Is this hard to do? Any suggestions?