You guys may already have one, but whatever. This DIY covers the replacement of bank 2 sensor 1. Everything is done from the top of the engine.

FRONT OF VEHICLE

LEFT | RIGHT
B2S1 | B1S1
CAT | CAT
B2S2 | B1S2

BACK OF ENGINE

Tools needed:
Antiseeze grease
7/8 inch wrench
Breaker bar. Mine is 24"
torx head drill bits and a 1/4 inch socket to place them in
Dental pic or equivelant. (something small and pointy)







Disconnect the negative batter cable. Remove the hose to the coolant tank and plug the hose and tank. Remove the tank and place it as pictured.





Remove the torx head screws. Use a dental pic to pry up on the locking tab for each harness while pulling the wire strongly away from the coil. It's tricky but you'll get it. You can secure the wire with a ziptie as pictured.





Remove the sensor harnesses. Notice the tools I'm using to remove them, and where to poke them. It can be tricky. Get the harness out of the way. You can now see the B2S2.







Pull the B2S1 sensor plug forward and unplug it. Unbutton the heat shield that is around the sensor wire.







If you are replacing the B2S2 as well, you must cut the wire so that you can slip on the 7/8 wrench. Torque it off using the breaker bar. The B2S1 sensor is in the front. Cut the wire at the sensor so that you can slip over the wrench. Look at the picture to see how I torqued it off.





Note the two different harnesses. The one on the left is the S1, and the right one is S2. Pull the old sensor wire or wires out. Because they are cut, they will pull out easily. You may re-route the new sensor wire the way you like and mount it using zipties, but don't forget to add the button-up heat shield. Apply anti-seize grease to the threads as well.

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Excellent post! Kudos!

 

This is a really helpful writeup, thanks man. How much time did it take to swap these?

 

My mechanic charged me for about 70 minutes to do the downstream left sensor with my own $60 part. It didn't fix my code problem so I'll have to do the entire catalytic converter now. RockAuto had the best price I could find on sensors and converter.

 

I had CEL with Oxygen (Lambda) Sensor B2 S1: Heating Circuit: Malfunction / Open Circuit P0050 (audi code 16434) I started with diagnostics following Audi service manual. Did resistance test between terminals 3 and 4 of the sensor connector (audi manual was calling for 2.5 to 10 Ohms) my multimeter showed 0L. I purchased new Bosh sensor from ECS tunning for $170 (plus unexpected $55 customs fee on delivery damn(!))
It took me 10 hrs to remove and install O2 sensor before catalytic converter B2S1, partially because I am novice, and also because I didn't follow cleft asunder's instructions to the letter. Since I was trying different methods to do that I wanted to share my experiences.
First my wifes 2003 Audi A4 is 3.0 AVK engine, has 143K kms on it.
Be careful with plastic electrical connectors from Audi. I broke small pry tab on coolant level sensor connector because I was prying it incorrectly. After It broke, I use dental pick method used in above DIY and it worked.
Once I removed everything out of the way to the O2 sensor, I took a moment to think "why the drunk hell did Audi engineers installed upstream O2 sensor so close to engine block and at the angle instead of vertically"
So here we go. The sensor is 22 mm size. 7/8" fits, but it's looser. I tried to remove sensor without cutting it's wire, I had crawfoot 22 mm wrench, no space to attach any breaker bar or ratchet, tried u-joint, not enough space. Then I had crawfoot style O2 sensor socket, because it was hex style, once put in, it was already touching heat shield of the catalytic converter, so It wasn't possible to move it.
I ground off short 22 mm wrench to make it fit between engine block and cat. I tried to use combination of another wrench to get leverage, not enough space, was very close though. After loosing about 5 hours on different methods, I cut the sensor wire and used the round side of open wrench and pried it off with pry bar using cat bolt on driver's fender side. After 30 seconds the sensor was out.
Be careful not to break other sensors when you pry on the wrench. I accidently chipped plastic sensor's housing that is located after ignition coils, luckily, didn't damage much.
Be very careful when cutting sensor wire, there is another smaller diameter wire running close to that sensor, in my case, If I didn't check, I would have cut both, and that would make for a very bad day. I think the other wire was possibly for speed sensor, it looked like it was going into the transmission vertically. When you cut the sensor wire, try cutting it after the plastic ribbed section that goes for few inches from the sensor. This will make it easier to pull out the old wire out. In my case it was quite tight to pull it out.
I used craw foot style O2 sensor socket to torque new sensor. I was able to fit 1/4" ratchet to tighten it.
Finally, when the moment of truth came, connected the battery, started the engine and warmed it up, the ESP light was on... It must be, when you turn off the battery for longer time, as it turned itself off after few seconds when driving. The CEL is gone, both of my fuel banks show closed loop, before when CEL was on one was showing open.