So again to further elaborate on this problem I've had for a couple months now. I finally got down to believing that my O2 sensor could be the possible problem I have 147hp/162tq on a full bolt-on car when it used to be 165/159 before the header and downpipe and highflow cat. During installation it is very well possible that the O2 sensor could have gotten messed up or something of that nature. Now what should I do to determine if it really is that O2 sensor or not? I have no idea who I should take it to or if I can just look at it myself and determine that it is broken from just looking at it. And another thing, the car has given me a code that "the catalyst system efficiency for bank 1 is below threshold for the current engine operating conditions", there are 2 sensors on my downpipe and cat if I am correct, which one of them could be throwing that code the first or second.

 

that code is being thrown by the second.
it is the one testing the efficiency of your cat.

that code is very common with aftermarket cats. its nothing to worry about.