The car: 2008 audi a4 quattro s-line 2.0

I have a CEL po171. system to lean or somthing of the nature.
could mean, bad MAF sensor, Large vacume leak or bad o2 sensor. I know its an o2 sensor because Ive tested and cleaned the MAF sensor theres no signs of a vacume leak. The upstream o2 sensor test bad and I get no signal reading on Torque Pro. I changed the bad o2 sensor with the exact same bosh upstream that I took out of it. the car ran awesome. no rough idle and got roughly 8 mpg more. I drove approx. 100 miles that night with no problems, the next morning when I went to work the car ran lousy and got lousy gas milage not 5 miles down the road the CEL popped back on again po171.

could something be causing the o2 sensor to go bad, or (god forbid) the ECU be bad. if anyone has any input please let me know.