How to replace Catalytic Converters 2000 A4 B5 Avant 2.8 Quattro

[Werner Raschdorf]

The Patient is a 2000 A4 B5 Avant Quattro with 2.8 ltr V6.
Got a code from VAG COM which reads: 16805 - Warm Up Catalyst; Bank 1:
Efficiency Below Threshold
        P0421 - 35-10 -  -  - Intermittent
I've had the same code intermittently pop up for Bank 2 as well. Was told
by the dealer 2 years ago that both Cats are in need of replacement. Both
sections of flexpipe are worn through so they need replacement for sure
now. I bought the new cats and downpipes from Bosal and have looked at
Audiworld for help in how to replace these myself. The left side on the
drivers side does not appear impossible to reach with some contortion.
There are 3 bolts that hold the downpipe onto the exhaust manifold. The
passenger side however looks impossible to reach those 3 bolts. Any
suggestions from the experts as to how one can reach those bolts and from
where? Also, the O2 sensors are not showing any faults on VAG COM. Can they
be reused or do they have to be replaced at the same time? I am also
installing a Miltek Cat-back exhaust system after I get the new Cats
installed.
Any help would be appreciated!
Werner