Four months ago I purchased a new A6 Ultra, since then it has been back into the garage 4 times, each time for about 10 days with the same fault, the yellow engine management warning light keeps coming on, apparently its a problem in the Ad blue system. Has anyone else had this problem ? as I am beginning to regret buying the car, and I want the dealership to replace the car, but as expected they don't appear keen to do this.