hey i have an evolution with buscher racing cat back exhuaust, manual turbo boost and klm intake filter, the check engine lite is on took it in and they think it might be from the o2 sensorat at the new cats. Also when cold get some hesitation, any advice?

 

All you need to do is call Buschur and they will help you out and make sure you don't have a problem with your product. They have good customer service.
As from what I have heard some people have CELL problems and some dont but every car is alittle different and if you are running the CAT you honestly shouldn't be gettting this. Also if you took the car in they should of been able to identify the CODE that was thrown for the rear O2 when they pulled it. I would get some evidence and make sure to identify the code then if it does result in being the rear O2 then contact Buschur. I am saying this because several people are getting CELLS due to Knock or fuel cut. I don't think it takes much to set the sensor off on the EVO. I have a Cell as well on my Buschur turbo back as well but I run with no cat.