2003 A6 V6 automatic (no turbo, no quatro)
70,400 miles (occasionally, during cold season, I use standard grade gasoline).
Check engine on, stays on (car runs and drives normally, fuel consumption normal).
I use ross-tech cable and software to read the code; it shows:

18407 - friction too high; exhaust cam adjustment bank 2
P1999-002 - lower limit exceeded - intermittent

I got this hint from my friend, engineer working for Acura in CT, so before doing anything else, fill and use 2 tanks of premium grade gasoline.
It worked for me - check engine off and stays off.