O2 sensor troubles

I just put a new motor in my 93 and also put in a new O2 sensor. Now, after only 2 oil changes, I'm getting crappy mileage and the computer is saying that the O2 sensor is stuck on lean. What would cause it to go bad after such a short time. Is there warranty on these things?

I had a look in the back, and the air hose that runs to the carb from the manifold is off. Must be to warm the carb or something? Would that do something to the mixture?

Sure, it would make it run lean.

Not the vacuum hose, but the big silver air hose. A mechanic told me that it was just to warm the throttle body for a bit when you first start it.