There is nothing in the A6 hole on that model as the Motronic engine management system does both injection and ignition. As far as not being able to reset the codes, it should be a matter of reading all of the codes until you have read the first one twice, ensuring that you read them all and then reset. Every now and then I will get a pesky one that I have to do right by the clock. By this I mean I have to stand there and watch the second hand sweep 5 seconds, release, light comes back on after 5 seconds and then hold the button down while watching the second hand sweep another 5 seconds. I know it sure sounds like over kill but I have had to do many times this way, but most of the time you can just count off "one-one thousand, two-...." and it will reset fine. Double check and make sure that you have no other codes stored. What codes is it storing by the way? As far as the functions of A7, it actually has modes 1,2,4,5,&6 and they have to do with fault codes, service light resets, data rate transfers, setting service intervals, check for odo manipulation, and a multitude of other things that are all related to the instrument cluster.