Volvo: Volvo S40 2000 oxygen sensor, oxygen sensors, volvo dealer

Question
I bought a Volvo S40 2000 and I found out from Volvo dealer repair shop that
my car's check engine light is on because I need to change my front oxygen
sensor. I already bought it, so I just need the exact location of those front
Oxygen sensors and step by step procedure on how to change it myself. Volvo
dealer said that they don't be sure If they change de sensor, the problem will be
resolve. Thanks in advance. Have a nice day. I hope you can help me.

Answer
Nad,
It is in the exhaust system in front of the cat converter. Follow the wire up and disconnect it and unscrew the sensor.
There is no better discription of the replacement being that all the manuals assume that a mechanic is doing the work, and has certain mechanical skills.
O2 sensors sometimes need to be replaced in pairs, that might be what the dealer is saying, or it is the first step in making the diagnosis.