Replacing oxygen sensor

Has anybody replaced an oxygen sensor on a 2000 Audi A6 2.7t. How hard is it to replace, I beleive the oxygen sensor is before the cat.

AS I understand it, it MUST be before the CAT, because it measures unburnt fuels, and the CAT helps to filter the exhaust. It should be right behind the exhause manifold. I can look for it on the repair CD tomorrow if you need me to. (its in my work computer right now.)

There should also be one after the cats. All cars made after 1995 are OBDII cars which monitor the gases after the cat with an additional O2 sensor(s).


Andy