I have a 1995 Volvo 960 base sedan. The check engine light has been on for about a month or so now, and I was sure that it would be an automatic failure at emissions. However, they didn't even look at the light, but failed the car because one of the nipples connecting a vacuum line to the engine has corroded away. This was an easy fix; I used a piece of hose the Volvo dealership gave to me to fix this. The car later passed inspection.
I just figured out last night how to use the diagnostic system and got the code 2-1-2, for the oxygen sensor. I have read that one cause for this code can also be a vacuum leak. Since this is now fixed, I figured I could reset the code, but it stays on, which leads me to believe that the oxygen sensor itself is having problems. My question is 1. Where is the oxygen sensor on my car and 2. What is the best way to access it to replace it, if that's what I need to do?
I don't notice any issues when driving. It still runs as good as in '95 when I used to have to sit in the back seat when my dad drove.
Thank you in advance for your help.

 

I just changed the upstream o2 sensor and going to change the rear. They are both located right about under the passenger seat where the cat is. You'll see one in front, and one behind it.

They're not cheap though so I would confirm that it's really your o2 sensor (go to a auto store and see if you can get the code read). Also you need to make sure if it's the upstream or downstream o2 sensor that is bad.

To change it, you'll also want something like this to remove the o2 sensor since space is limited around it and the opening allows you to slip it over the o2 sensor around the wire:

 

update:
thankfully i didn't need to mess with the o2 sensor as the vacuum line i fixed was the actual problem. i was just incorrectly resetting the fault code.