I finally had my valve stem seals replaced. Now to remove the clogged CAT. I've had a look and there seems to be two oxygen sensors. One before and one after the cat. So the question is, if I remove the CAT, what effect will it have on the engine management system. Also, the exhaust system seems to be original for my '97 model so the likelihood of breaking a sensor when removing is also there. What do I replace it with if the worst happens?