Ok..well I just put a dc header on my car...before this...with the stock manifold...I did not have my o2 plugged in...now..with the new header..I had a spot made for it and am using it now...made my engine light go out...buuuut...now from like 2-4 thousand rpms....the car hesitates to pick up speed...like something is holding it back.....a friend of mine..said that this happened to someone he knows....when they finally got their o2 hooked up...other than that...runs and picks up speed great past that point in the rpms.....has this happened to anyone else? Ill probably just unplug it and see if that helps...but why would the o2 sensor cause this?