The code is either the cat or the pre cat. If you're just at 100K you might want to speak to the dealer they may fully or partially cover it under emissions warranty if your engine is stock.
Mine does the same thing @ 115k so I'm expecting a 500-1000 bill next time I go for inspection. I might wait till after I fail as I can get an exemption in Mass if I spend enough on trying to fix it, in case there is other problems, but I like the car so I may just get it done right.
Does anyone know if this is a common problem on 2002 Protege5's?