my buddy just grabbed a high flow cat, then he tried to stick the 02 sensor back in and hook it up but then cat is meant to be upside down so then the 02 sensor is sticking downward and could easily hit the ground.
He was riding around with the 02 sensor clipped up so it wont hit the ground and we just threw a bolt in the hole, taped up the 02 sensor hole of where it comes out.
I just smashed in it after 1 day and now the engine check came on did it break becase i smashed to hard or becasue the 02 sensor isnt there, and is there anyway he can make the highflow cat work because it does sound better.

 

the secondary o2 sensor measures the difference between the primary heated o2 sensor in the header. if the values arent correct it throws a CEL 67 saying the cat is bad. the secondary o2 sensor has no affect on the car so you can either deal with the CEL or put it back in the cat and reset the ecu

 

I believe that your car will run lean and you could wash out your rings w/o the second o2 sensor.

I installed a test pipe and purchased O2 sim chip and soldered it to the ecu and wrapped a peice of foil around the O2 senor and zip tied it to the linkage.
I no longer throws any codes.