[JL8Jeff]

The real problem is that most exhaust shops won't touch replacing a cat if there isn't a legit reason to replace it(ie the cat has high mileage or it's clogged or it melted). If you cut out the old cat and clamp it in and go to an exhaust shop, they will gladly weld it up for you since you were the one that replaced it and they won't be liable. That extra air tube is there for some vehicles that pump fresh air directly into the cat to help it heat up and function faster. It has nothing to do with OBDI vs OBDII.