My 2002 A6 petrol estate started running badly at 82000 miles with the automatic gear change around 2000rpm causing a sensation like a missed beat and I wondered about spark plugs. It ran smoothly above 2200rpm at sustained speed but in traffic it was lumpy to drive with the frequent changes around 2k My trusted VAG reader gave the code in the title saying:

18034 Powertrain Data Bus: missing message from TCU : P1626

The solution took hours to track down across multiple Audi forums so I wanted to share it to maybe save others some time (and money). 

Change the transmission fluid (ATF) and pan.

The 80k service requires this and I’d missed it having only recently bought the car. The garage told me the black burnt smelling stuff that came out was a long way from the green replacement. Now it runs as smooth as silk. Total cost £280 (Sept 2024)
 

There are multiple requests for help on this out there and buried in some replies is the suggestion to change the ATF. This was only confirmed by one user in the searching I did. Thanks to him for posting, I just thought I’d add some confirmation that his suggestion solved my problem. Just wish I’d noted the forum and member’s name. Hope this turns out to be useful for someone else. 👍

Edited October 2, 20241 yr by Bottle
Addition to topic line, added tags and details of car.

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