Another 420 question

Ok I know this question has been beaten to death , but mine coded and about that same
time I noticed a burning smell. So I've been told that the cat. might smell like rotten eggs
or sulfur if it is burning up, mine smells like burning plastic. With 150xxx miles on it who ever owned it before me took good care of it. Underneath it is real clean and the exhaust system looks almost new, no rust at all. I have looked around and can't find anything that would make it smell like that . The truck runs the same as when I bought it, and I hate to say it but this has been like this for over 6 months. The scanner shows the OS2 bouncing around a little. I still need to check the oxygen sensor but that still is not going to narrow down that smell. The scanner says that everything is ok except for the 420 code. This is a California vehicle and I hate having to smog it. I took it to a couple of places but because of the weird smell both mechanics didn't want to touch it. I can't afford to take something like that to a dealer, any ideas. ( 96 2.4 )
Thx