2003 A6 V6 automatic (no turbo, no quattro)
71,700 miles

Check engine light on.
Car starts and runs o.k.

Checking with ross-tech cable and software shows:
17528 - oxygen (Lambda) sensor heating; B2 S1
P1120-004 - open circuit - intermittent.

To replace the part, I used following spare parts and tools:

Bosch Oxygen sensor part #17113 (with connector), purchased from Auto Zone (online) for $106.99
Oxygen sensor wrench - item "powerbuilt 648691 oxygen sensor offset puller", purchased from amazon for $7.61.
I used a short (5" - 6" long) ratchet wrench to have a bigger rotation angle in the space available.
All other tools are standard (metric).
I open/close the coolant tank cap (with cold engine) to release any remaining presure from the system.
Move aside the coolant tank by disassembling one hose (red arrow) and 2 screws (green arrows):



Disconnect and move aside ignition coils connector and 2 camshaft position sensor connectors:

to be continued (I can upload only 5 pics at a time).

 

2003 A6 V6 (no tutbo, no quattro) continuation:
Remove the cover of the oxygen sensor connector.
I post the photo to see the "locking"; lift the cover aprox. 3/8", incline it to the front of the car and lift up. I had to bend it to figure out this .

Disconnect the sensor connector:

Observe the routing of the cable to be able to reproduce it and avoid future rubbing/burning of the cable.
The B2 S1 oxygen sensor is located, between the body and the flange of the catalytic convertor:



Observe all the standard safety rules, you know what I am talking about - battery, hot/corrosive/irritant liquids, protection eye glasses and gloves, heavy objects, and so on...
Happy DIY !