A couple of weeks ago I had to replace my fuel filter. Some time after that my A6 had the CEL come on. After plugging in my code reader it said P0303 misfire or words to that effect. Since I had an issue with the fuel filter I attributed to a clogged injector. I ran a couple bottles of Techron Fuel Injector cleaner and after a couple of days the CEL cleared. The car ran better after the fuel injector cleaner but took a number of starts to clear the code. I would recommend the Techron product if you have a similar issue. Cheaper than the other possibilities of fuel injector itself, ICM or spark plug wires.