91 Park Ave Catalytic Conv. Problem??

[DrWatson]

Hey all,
I have a '91 3.8L and was driving along when all of a sudden it started driving rough and kind of hesitant like. When I got home (about a 1/2 mile) later the catalytic Conv. was glowing red. I figured it was clogged and changed it. Still runs the same way and from a cold start in two minutes its glowing again. I pulled the codes and got a code 23. Local places giving me different views including the dealer. MAT sensor but most (the dealer) agree there isn't one on this car. Anyone have any ideas? All help greatly appreciated.