This thread really saved me!! Have a 2017 allroad a4. Audi didn’t want to know, told me to book a repair which was 5 weeks away. I managed to get the part £90 and with some reassurance from a local Audi specialist that it was an easy fix, decided that I was going to try and fix it. Well it was a 10 minute job, warning lights for brake servo and start stop all went off. It was £150 for Audi to put it through their computer which they HAVE to do funny enough. Word of advice - as soon as this brake servo warning comes on, find a safe spot and don’t drive it any more, you risk busting your ABS which is pricey. Thanks very much.