Chevrolet Repair: P0420 Trouble code, astro van, trouble code

Question
I keep getting this trouble code on my 98 astro van, what are the likely causes, and how would you check them out?

Answer
Hi John, This code would tell me that your cat con is not working. You need to replace the cat con to stop this code from coming back. The part is very costly for this repair but if your under 80k it should be under factory warranty. Call your local dealer for this info.

       Let me know how you make out         Mark