So I had a limp mode problem with my 2003 A4, after many garages and mechanics looking at it I’ve been told to get everything from a transmission fluid change to a new gearbox, however I decided to do some research on my own and I found that it was likely a component called a transmission range sensor that was putting it in limp mode. I found an after market one and had it fitted, that was about a year ago and it hasn’t gone into limp mode since.
hope this is some help.