I have a 2000 a6 2.8L and have a P1423 (bank 1 air flow too low) code for a while that won't go away. I've changed the o2 sensor and had any bad vac lines replaced. Was told it might be the kombi valve that might be stuck or filled with carbon deposits. I noticed it's in a pretty tight spot. Is there an easy way to get it out to look at it to see if it needs replacing? Or do you think if I run Seafoam through the system that it would clean in out?

 

It's tight but you can just weasel it out of there. If it truly is your kombi then you'll need to replace it too late for seafoam.
Buy yourself a vacuum pump at the auto store and hook it to that Kombi valve and listen for it to open up.

 

The Combination Valve at the back of the head? Start by removing all the wiring harnesses, vacuum lines, etc, that you can from around the valve, then, amazingly, the room opens up and its not so bad! You can actually get an air wrench into one of the three bolts and a socket into the other two! I found this out from having to do this three times in one week (and my hands STILL have not healed!).

Bob