fault 18034, p1626 stalling/ rough idle

I would appreciate any pointers for a novice diy user.
I have a repeated couple of engine fault codes mainly 16891 idle control too high - sporadic, 17544 system too lean, and 17705 pressure drop between turbo and throttle valve.
I have trawelled through the posts and figured it was nothing serious so keep resetting codes.
NOW the car (A4 1.8t avant) idles like a dog and sometimes cuts out, not good in an auto!!
The new code is 18034 powertrain databus missing message p1626 this code comes straight back after clearing and firing up engine.

NOT HAPPY....
thanks for looking , and hopefully advising!!