First off you need to find out exactly which code it is setting. There are MANY codes that involve the o2 sensors and cat converter. All are diagnoised differently. If you eliminate the converter you will have a constant CEL. This is true with all 96 and up ODB-2 vehicles with a 02 sensor downstream of the cat. If I were guessing though, I would say the cat is coming apart. A simple test you can do is, thump on the outside of the cat with a mallet. If you hear crap rattling around inside while you hit it it is most likely junk. Of course, do this with the exhaust is not hot so you wont get burned.