P0421 code please help?

Hope somebody can help me with this one. The check engine light went on 3 days after I bought my car from a toyota dealer. I took it to the dealer and they told me it was the catalytic that was probably dirty. But since it was a used car they couldnt fix anything. I didnt feel any major problem with the car, it runs fine, so I didnt bother to get it checked. But few weeks ago a friend got a car just like mine but his seems to have alot more power. So I thought maybe something was wrong with mine, so I took it to autozone, they checked it and the P0421 code came out, they told me it was one of these three that was causing the problem. Bad catalytic converter, a bad rear oxygen sensor, or an exhaust leak.

Sometimes in the morning a whistling sound comes I think from under the car but when I step on the gas it goes off, if I let go of the gas it comes on again. After a while it goes off completely so maybe I my catalytic is plugged? Does anybody know how can I check which of those three could be causing the problem so I can go to a mechanic and tell him specifically what to fix, they charge me 50 dollars to put the OBD ii to see what's wrong and I already know what code is going to come out. I have a mitsubishi lancer OZ rally 2005