02 Sensor Check engine light 97 Geo

Hello,
I was at a local auto parts store to buy a new O2 Sensor. The parts book says there is an "Up Stream" & "Down Stream O2 Sensor" in my vehicle.
I would like to change the O2 Sensor that is right in front of the engine.
Which O2 sensor do I purchase?? :eek7:
The Up Stream or the down stream??
Thanx

Upstream in the one you want.

The context of the term they use is up-stream of (i.e. before) the Cat. converter. The one before and after the Cat are used to assess the Cat. efficiency.

I've experienced a P0420 code I need to clear consistently with my Geo, but there is no perceptible impact on emissions. My mechanic says one solution the manufacturers use is to re-map the ECU to not throw this code, but clearing the code is easy enough...