Had the same message about brakes servo and start and stop… called Audi roadside service and they found the fault… sent the information to Audi… drove to Audi next day and they changed the sensor for free, under extended warranty… car looks and feels fine to drive… had problems with ad blue unit in the first 3 months with the car (bought used from Audi) and they replaced it also without complain… until now, happy with customer service…