I have a 02 A4 3.0 and it goes into D but when I drive it wont shift. So I have to put it in S and that waste a lot of gas. Any ideas on what it could be?

 

You gotta post the fault codes, there are a few things that can cause that issue, TCM is malfunctiong, shifter selector switch isn't seeing "D" causing it to go into limp mode ect. If the car isn't shifting in drive there is more than likely gonna be a fault code.