Next time you go to the dealer ask them to make sure your car has all the 12V battery updates. There was a customer satisfaction letter that was sent out about it but I forget the number. I have a 2013 SEL and Ford kept telling me that my 12V battery was good but I started to have issues with the smart key not unlocking the door. In my opinion it was worth the $200 (parts + labor) to let them do it. The battery alone costs at least $120 on sale from Motorcraft - it was going to cost $150 on Amazon + $30 shipping so why not just let them do it considering it's hidden under the floor in the back. I did replace the 12V battery in my Prius and the procedure is about the same, not very much fun outside in 4F weather. So between replacing the 12V and the coin cells in the FOBs no more issue. I had already replaced FOB battery a few months prior to having the unlock problem so I knew it wasn't that.
Hopefully you already know about the emergency key that is hidden in the FOB? Useful if the battery dies and you can't get the car unlocked.
Edited by jestevens, 05 February 2018 - 07:31 AM.