I need to replace an o2 sensor! How can I tell which sensor needs fixed? on 2001 Audi A6 Quattro

I'm a college student thats stuck w/ an expensive car! ugh. i love my baby, but she needs a new o2 sensor. the Mechanic was going to charge me 80 to diagnosis which sensor, 100 for labor, and 100 or more for the part. How can I find the faulty sensor myself? I've seen the parts online for around 100. Is this a job I can attempt myself?

Asked by for the 2001 Audi A6 Quattro
have it scan tested for codes it should tell you which one it is
Thanks for the response! Where can I get scan tested, other than a mechanic who will charge me 80 dollars.
1 more answer
take it to your local parts store they will scan the code for free. They will know what sensor it is by the code. Upstream is before the convertor and down stream is after it. You will need a 7/8" wrench to remove the sensor unplug the wire and rplace it in reverse order. Hope this helps you out, now quit worrying and go get us some good grades!!!