Good evening Vianney, I have a 2008 S4 Aero and coincidentally my car failed to start last week. I had driven into town and when I returned to start the car, turned the key, pressed clutch and then the start button nothing but a faint buzz from under the bonnet. After several attempts to start it, about twenty minutes later, it fired up. When I returned home I tried to start the car at various intervals throughout the day and it started evey time. Later that evening I had a look around under the bonnet and to my absolute amazement I found the live cable to the battery terminal was just pushed into the socket and not properly crimped. I have now replaced both terminals and fingers crossed that should be a cure. Could be worth you checking yours. All the best Tony.
