I would like to gut the cat on my '05 b6 1.8tq 6-spd......wanna know if any one did that to theres, if so what did you do with the second o2 sensor, the one after the cat.

 

Seen it done before and it just caused the engine magment ligh to be permintly on and not most efective running. The second lambda probe is compering the result to the result from the first lambda before the CAT this measurent will then be feed back to the engine magment ECU which will help to decide how much fuel to keep injecting. If you remove the CAT the diffrence won't be the correct measurment that the ECU is epecting and concercently mess up your fueling.

 

There are replacement sensors available for the rear o2 that send a false signal to the ecm telling it everything is ok. That ways theres no cel. I wouldnt really bother gutting your cat considering you wont see much gain from it.