On 2015 and prior the fuel door doesn't lock. On the 2016 models when the car is locked the fuel door will not open. I tried opening the fuel door on a locked car at the dealership. Wouldn't open. This is important for me because of the MKX's capless fuel filler opening. If I can't lock the door then I insist on using a locking gas cap, which won't work with the Ford design.