A little less than a year ago I had the alternator replaced on my A6 quattro 2006 3.2L. Ever since that time no diagnostic codes are stored in memory, although I have had plenty of problems with vacuum leaks, rough idle, voltage going up and down when the engine is cold, etc ...

Is there something that has to be reset to collect and store diagnostic codes? Without the codes I don't even know what to look at.

Thanks.