CODE PO420

I have a 2000 Z24 Chevy Cavalier with 113,000 miles on it.  A couple of weeks ago my check engine light came on.  I got a diagnostic test done and was took I have a bad CAT.  No problem I order one and have a shop place it on there.  60 miles later my check engine light comes back one.  Take it back to the shop and was told its code PO420 the new CAT is defective.  So I had the shop order a good one for me and install that.  60 miles later my check engine light comes on.  I take it back to the shop and they are telling me it's still pointing to the converter.  They replaced both 02 sensors with new ones to see if it would do the trick and nothing happened.  I still have a check engine light on after a diagnostic test said my original, my new one and the new one from the shop is bad.  And the 02 sensors didn't make a difference.  What could it possible be?

If the car is running overly rich, smoking, over heating, leaking antifreeze into the combustion or any thing in the exhaust, it will coat the catalyst and ruin the converter.  Good luck

I'm told the car is running rich but its not smoking, over heating, leaking antifreeze into the combustion or anything i the exhaust.  I am at a lost on what to do next.