Audiology

I just bought an '06 A4 2.0T with 41K miles. When I had it checked over by my mechanic, everything came out fine. They checked codes and said the only one that came up was P0455, Evap system large leak. They said they usually don't worry about that one because it is most often a loose or improperly seated gas cap.

Well, 6 days later, the code came back and the Check Engine light is on! I won't be able to pass inspection with this problem! The dealer is quite amenable to fixing problems (it's the second car I've bought from him), and we have a used car lemon law, so I am not terribly concerned that this will cost me a lot of money, but it will cost me precious time.

Here is my question: can I swap gas caps between my 2007 A4 3.2L with the 2006 A4 2.0T to troubleshoot this problem? The caps look identical (bright green threads), but when I do part searches on Amazon or Ebay, they come up as incompatible. Are they actually different?? The two cars are different engines and one year apart, but otherwise pretty similar.

Thanks for any advice!