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I suspect it can be refurbished. Your best bet is to speak to an audi specialist or a gearbox/diff specialist.

Hi Andrew, I got my master cylinder done today and as per the engineers instructions there is a definite sequence to observed before bleeding it out, to my method I would have just fitted it, bled it, and then tried to start the car, wrong according to our instructions, before you bleed the unit out you must leave the pedal on the floor and then start the car run it for a couple of min then turn it off and bleed it out, looking at it logically think that by the pedal being at the furthest point of travel the sensor at the top of the cylinder registers itself electromagnetically like a Hall sensor and despite my mate snapping the sensor mounting, we had to re mount it and fix it on with cable ties but it still works until I get another cylinder. I have asked the engineer if he can get the voltages for the diagram supplied, he says its not his speciality but he will ask the electrician where he works to look and advise on voltages, to my mind the control unit is blocking the introduction of the new unit as you described you can see the data bits change but its not allowing the the cylinder to register to start the vehicle, I don't honestly think its a wiring problem but it won't hurt to try another piece of loom, I could well be wrong and that solves the problem, also before fitting the new unit we deleted any codes that were present, back to you in a couple of days, it was the master cylinder not releasing the pressure that was causing my clutch to slip, why are these cars so complex. Steve.

An update from me too. I ordered the sensor (5Q0 906 207 B) from Cardiff Audi on Friday and it arrived this morning. It took a few minutes to remove the old sensor (I think I was being too careful not to break anything off) - the hardest part for me was to unhook the latch on the sides, but installing the new one was super easy. Both brake servo and stop/start warnings disappeared immediately after the sensor was replaced. All thanks to everyone on this thread and especially Ambrose whose posts I found extremely informative! Obviously I'm not impressed that these parts are failing in the first place (or that they could lead to bigger safety problems), but I am hugely impressed by everyone on this forum coming together to help others. I would have been without a car and few thousand pounds poorer otherwise!

Hi guys, As promised, here is a short video of how to replace the Brake servo sensor of an Audi A4 B9. Not the best or clearest video, but I hope it helps. Let me know if you have any questions. Ambrose

Hi all, I had the same issue where the brake servo and stop start error appeared. After reading this helpful thread, I thought I would do my part to help out too. 🙂 Issue stared on the 5th June 2021 and it was fixed on the 9th June 2021. The car has been driven for approx. 100 miles during this time. I've called up Cardiff Audi and fortunately they have one sensor left in stock. Please note that 5Q 0906207, 5Q 0906207A, 5Q 0906207B are all cross compatible as confirmed by Audi parts department. But of course best to get the latest B version if available. Please note VW, Seat & Sokda all share the same part and I've checked with my local dealerships and they are all the same price £79.60. I've managed to get it down to £76 at Cardiff Audi. If unavailable, usually you can order it and it takes a day to arrive if ordered by mid-day. Installation will take less than 5 mins if you are unfamiliar with the car. I have enclosed a print screen of the Audi A4 Shop manual. Link are here: https://cardiagn.com/audi-a4-b9-8w-brake-system/ Page 146. I can do a simple video to demonstrate this if anyone wanted. One thing to be noted is when you unplugged the sensor, some pressure will be released from the hole. I've asked Cardiff Audi whether this falls part of the recall part and they said no. But they've done a few of these jobs and knew about the issue. I hope the above helps. Kind regards, Ambrose

Hi I used to have the same problem, happened after the stronic oil change apparantly (as this is what the garage said) an o ring (or something on the gearbox cooler) has snapped when retightening and there was a tiny oil leak when in D and not driving (something to do with pressure) they changed it (cost £6 from Audi, again this is what they said - I didn’t pay they accepted they had messed it up...) the smell to a while to disappear but it did eventually after a few month... it could also be the DPF regen smell, or both... get I checked by the garage where you had the oil change done... hope this helps