If 'running rich', the cat is not the main issue, unless by that you mean the HC emissions are high. Does this year use an O2 sensor? What happens to the HC emissions if it is disconnected? Sounds like you may be running 'open-loop', or 'limp-mode'.

If you are running lean, then I WOULD suspect the cat. However, if the last one lasted only two years, then I would also suspect another problem - like burning oil.

Steve