Hi, I believe this is a serious and emerging issue for Audi and us owners.
I count around x26 owners with this problem on this forum alone, I'm 27th and counting. My A4 Avant Ultrasport 66 reg has 30k miles on the clock. It sat for weeks from March until end of June with intermittent short journeys to shops etc. The brake servo restricted warning came on around mid/late July I believe. I contacted my local mechanic, but couldn't get through for a week or so. The warning then changed a week or so later to Auto Start/Stop not functioning as well as the brake servo issue. I then decided to contact Isaac Agnew Audi (the only approved Audi new car dealer in Northern Ireland). I told them the issue and they said we can book you in on 31st Aug and run diagnostic for £80 - that was around two weeks away. I decided I couldn't wait this long and contacted my mechanic again. This time I got through and he took me in the next week.
Long story short, he replaced the faulty brake sensor (it seems it is now a slightly different p/n from possibly a different manufacturer!), he also charged up a lowish battery and got all error codes cleared apart from an ABS fault code (all for £80, I might add). He told me to take it to Agnew's, so when I contacted them this time they said no problem come in 8th Sep, but changed their minds (for some reason) and said come in 31st Aug (the original date they gave me). Agnew's at no time told me the original error was a ticking time bomb, otherwise they should have brought me as soon as I first contacted them. They took the car in and got back same day with a repair bill of £2,708 for the ABS and also in addition a front suspension part (a suspension part failing at 30k miles - sounds like my dad talking about Fords in the 1970s!).
I can't afford to pay £3k for a car with only 30k miles on the clock, or the next time, whenever this fault decides to come back!!
It seems the faulty sensor is misread by the computer as a hydraulic problem and tells the ABS to compensate. What it doesn't inform the owner is that this also in a short time destroys the ABS unit, which needs replaced:- Audi should at least have forced the car to go into limp mode. As a design engineer, a risk/fault assessment should take into account faulty sensors (737 MAX crashes are an extreme example) and rather than conclude a perfectly good hydraulic brake system was the problem cause the ABS unit to self-destruct it should recognise this as a minor fault and not do irreparable damage. Of course Agnew's will not offer a goodwill gesture as I bought the car from Oakwood Bury, and therefore Audi UK will not offer goodwill either. Both have large budgets but are not willing to do anything for both theirs and their manufacturers plummeting reputation, and it is you and I who pay the cost of Audi's design fault.
My car is still with Agnew's who haven't got in touch with me for a week, I've been the one checking up on updates (same with Audi UK, although to be fair the girl I spoke with (Ami) was trying to be sympathetic and helpful, but didn't seem to have anything else to offer).
I can sort of understand Agnew's and Audi's stance regarding goodwill, both have a budget and it is up to the local dealer to decide if it's worth helping out a customer (or a new potential customer like me), however the bigger picture seems to escape them entirely.
If you are all fairly/exceedingly disgruntled (some have been fortunate to 'only' have to fork out £250 or so for a brake sensor replacement), you should submit complaints to Audi UK (call 0800 699 888 or email email@example.com) - if they get enough owners with this fault it should get raised with Audi and a recall may be in order - to my mind a reputable company knowing a fault can occur in a brake system, through a design error, must put their hands up and take responsibility. Someone could get killed if this causes brakes or ABS to fail!
Each of us alone aren't worth taking notice of, from what I can see, but together we must be listened to. I know Oakwood in Bury (2nd hand Audi dealer - seem good guys) have now had x4 Audi owners with the same problem (add that to the x27 of us A4 owners and a few A5/Q7 owners as well as those on other forums)!
If anyone else has a good idea of who to contact (Autocar, Motor Ombudsman, What Car etc.) please let us all know.
Anyone with me on this?
I've never had to write something like this before, I went from x10 worry free years of driving a Mazda 6 to Audi, thinking the premium we all pay for a car like this must be justified - now I'm just plain angry and no-one in Audi seems to give a stuff about us. Cars breaking down out-of-warranty is something we all accept, parts get old and wear etc, but we cannot accept a costly built-in design error like this and pay dearly for Audi's mistake.
Thanks for reading this to the end - let's hope Audi genuinely care about those who give them their hard earned cash!
PS:- The press may also mildly interested in this, especially a premium German car manufacturer with a pandemic related fault they aren't acknowledging - what do you think - keep it in reserve until Audi respond?