Vacuum leak will do it too from what i've heard. I donno for sure though.

Street Wolf, i'd try a new O2 and see what happens. Maybe a little RTV from the blower install made gases go out the exhaust to damage the O2 sensor? I've heard of that happening to people, even on brands of RTV considered sensor safe. That could be a possibility, i've had good luck though (knocks on wood). Maybe its just a fluke thing, either way I think your O2 is bad. Good luck, let me know what you figure out.