Engine starts and immediately stops when I release the key: FIXED
 

My S5 started doing this recently. Initially it would stay running on the 2nd or 3rd restart, then I would have to hold the key just right to keep it running and gradually got worse untill it would not continue to run at all when the key was released.
The problem was the ignition switch and it is easy to come to this conclusion because when the key is released, after start, all the dash lights would go out at the same time.
New ignition switches may be available at your favorite autoparts store or Rockauto.com. My local O'reillys had difficulty finding one and still hasn't got back to me even though their website lists several. You have to be careful to request one that matches your transmission (AT or MT) and if your car has airbags.
New ignition switchs go for between $100 - $150.
Used can be found for $50 in this forums classified pages.
Or you can repair the switch like I did for $0

They are very easy to pull apart. Only one screw holds it to the left side of the steering lock mechanism. A white plastic cover clips to the black piece where the wires are attached. Either disconnect the battery or disconnect the 4 wiring connectors from the switch so it can be moved to an accessable position before releasing the clips. AND DO NOT let it fly apart like I did when the last clip releases as all the guts and springs fly out. Best to hold the switch with the wires up and white cover down and gently ease apart. This way you'll see how it all fits together even though it can only go together one way. You will also see the position of the 6 springs. Once apart just clean all the contacts. I used sand paper on the worst contact. One of the contacts was black which was causing the problem.
This is not a difficult job at all and it functions perfectly with out spending $100+
I hope this helps someone keep their car going longer :)

Just like the window switches eh? Good info!

I'm a sucker for success stories lol

And he's right, those things will fly apart if you let them. I had to replace mine because I couldn't find a piece.