Which o2 sensor to replace?

[Dj.SammyOaks]

So a SES light popped on yesterday and I went and got the code scanned this morning and the guy said it was either the cat or o2 sensor (like i figured). But my 2000 grand am se (like all) has 2 o2 sensors, the upstream and downstream. which one do I need to replace? are they interchangeable? like if i buy one will it work in both spots if i mess up? anyways, which one should i replace or do i have to just kinda wing it.

 

[melsg5]

You have to provide the code to determine that.

 

[msmith1956]

Like Mel said, having the code will tell you, but from my experience, it's usually the upstream sensor that fails most of the time.