Wideband sensor issue, exhaust related?

[Mischif]

Right now my exhaust system is just the n/t manifold and thats it, no down pipe or nothing. I installed my old wideband sensor in the stock sensors location. It seems as if it reads fine for almost the first minute of the car running, then it bottoms out lean. Turn off car, start back up and does the same deal. Is this cause there isn't enough of a exhaust track after the sensor or is it just simply going bad?

 

[bling5tatus]

I would think this is a sign of a failing sensor. I'd try to calibrate it in open air again and see if it works properly then.