If you have an O2 sensor going, you should have a check engine light lit. FYI, there may be more than one O2 sensor on your chassis.
Usually, there is an O2 sensor for each side of the engine (would be located somewhere between the exhaust manifold and the point where the exhaust y's together into one pipe). Generally closer to the manifold side.
There may also be a rear O2 sensor that is plumbed into the single pipe located downstream of the catalytic converter(s) - assuming that it has cats that is.
Your check engine code will tell you left bank, right bank, or rear O2 sensor as far as which one isn't working. I will say that if you are going to replace one bank, go ahead and replace the other bank as well so that you have fresh ones on both sides.
R&R is straightforward. Put a wrench on it and remove. HOWEVER, they are often seized into the exhaust pipe, so liberal shots of a penetrating oil daily for a week or so prior to removal is a great idea. Sometimes, they are located in such a way that a special O2 socket has to be used for removal. They are available at any parts store.
Hope this helps.
