As was mentioned above, in many cases the O2 sensor will drift. This is IMHO more because of age than anything else.

I'd say that after 100K miles the upstream O2 sensor(s) should be replaced even if it's nothing more than prevention.

Rockauto is your friend when you do this.

I used to be pretty open to using whatever sensor I could buy for less on Ebay, but late-model cars have enough differences in their O2 sensors that I would try to get one specifically for my car.

If I'm all wet on this last point somebody please correct me. I'm talking specifically about the wideband sensors.

R.