I have an 08GSR intake turboback exhaust (no cat) and 3port boost control. When the car was tuned the rear O2 was not working, now if I replace that O2 is that going to effect the tune/performance of the car in any fashion? It runs fine as it is the only "issue" is the SES light from the o2. I work at a shop and scan it regularly to be on the safe side. Any help is appreciated.

 

Replacing the 02 sensors will have no effect on your tune. It will however most likely throw a code since you’re catless. Just buy a 02 spacer to fix the issue.

 

I bought a spacer already I was more concerend with since the car was tuned without the rear sensor that adding it would effect performance in some fashion.

 

The rear sensor is only used to verify a working cat. It has no effect on anything else. If you are not currently throwing a code, then your tuner has masked off the code and you do not need to replace the rear sensor. Just plug the hole and move on with life.