Grand Am GT 1997 Check engine light

my check engine light is on in my 1997 Grand Am GT 4 cyl (130,000 miles) . I had the exhaust fixed a few months back. I've been gone for over a month that the card didn't run can anyone help me with regards to what the problem might be, i think it might be a fouled oxygen sensor but I'm not sure. What does this usually cost to fix?

go to autozone or advance autoparts, they should scan your computer to find out what code you are throughing for free.