I'm replacing my exhaust, which is good and rusted in. I've managed to free the cat from the header, but how do I remove the O2 sensor from the cat? Is the rusty 19mm nut attached to the cat or the O2 sensor? I'd rather not damage anything. Also, I have a 3 wire O2 sensor, is this OE or the generic aftermarket unit? Either way, it seems to work... Thanks, Paul 87 4kq