as many have found 02 sensors on audi's go out much earlier than the 100k that bosch says they should last. i got a 4 wire universal for murrays i think it was ~$120. I also needed a set of ramps (~$35). The B1/S2 and B2/S2 sensors are fairly easy to replace, the S1's however look much more difficult (ive never done it so i can't say). First off is location of the sensors is B1 is passenger side and B2 is drivers. S1 is the front sensor (found on the headers) and S2 is the rears (found on the cats). As far as colors of the plugs, The green and brown are the rear, the black are the fronts. The black plugs correspond to the side of the sensor. 3 are on the passenger side and the last is on the drivers. Now for the pics (the rest is up to you :
<img src="http://pictureposter.audiworld.com/73987/b1_s1.jpg">
Above: The B1/S1 sensor
Below:The B2/S1 sensor
<img src="http://pictureposter.audiworld.com/73987/b2_s1.jpg">
Above:The B2/S1 sensor
Below:The B2/S2 sensor (sorry i dont have a pic of B1/S2 but its easy to find)
<img src="http://pictureposter.audiworld.com/73987/b2_s2.jpg">
Above: The B2/S2 sensor
Below: Plugs on the passenger side
<img src="http://pictureposter.audiworld.com/73987/plugs_1.jpg">
Above: Plugs on the passenger side
Below: Plug on the drivers side
<img src="http://pictureposter.audiworld.com/73987/plug2.jpg">
final tips: make sure you follow directions that came with your universal sensor when splicing the wires. The wires are held on along the tranny by little clips i pulled these out and more or less destroyed them so theyre not there, this hasnt caused any problems for me =). Good luck!
-AO

 

You can barely see the front O2's nevermind reach them =)

 

some people may not be able to because of no mechnical talent. i did them my 1st time in 3 hours, others can probably do it faster. s4 is probably harder then a6 or allroad because of space.

 

putting if off...but if you have a way, within 3 hours...I'll take care of mine this weekend...

 

once you remove maf housing,maf accordian hose, silver colored y-pipe, and coolant reservoir, which will take 1/2 hour tops, you can access the o2 sensors with an o2 sensor socket and some long extensions(2 foot).

you will need to get some reusable hose clamps for the 1 time use clamps on front of y-pipe to install y-pipe when you are done, some cable ties to fasten o2 sensor wires after installing new 1's(maybe, depends on how you route wires).

the hose that comes out of bottom of accordian maf hose just pops out and can be installed easier if you spray or apply some grease on it to reinstall.(hose is on 2001 and newer models i believe)

 

you can but a special tool to do the fronts...