I'm having a problem with my Cat too! I have a Honda Civic 2000 EX that my dad fixed up and played with, and he gave it to me around the beginning of last year. I just got my car back from the shop last week from needing my cat to be replaced again. It's the 5th time now. Luckily it's under warranty but this isn't something that I want to keep happening and I want to nip the cause of it. Every time I ask why the cat has failed, they tell me because it's clogged. I know there are a couple of various reasons as to why it would clog, but it's a bit beyond me right now. I'm sure my repair shop is tired of replacing it multiple times now too, but they're kind of fascinated at the same time since this never really happens. A part of me thinks it's something that my dad did to the car to cause this. Oddly enough, this has never happened to him when he had it. But he knew cars and didn't drive it as often as I do either. Help