I have an '03 Expedition. When it's very cold (teens or below), the front doors won't latch shut. The door lock mech sticks a bit at first, as well, at these temps. I've WD-40'd the door latch and tried to get some in the lock area itself--doesn't seem to help much. When the truck warms up enough that the doors warm (20 minutes or so), it does finally latch. If I slam the door hard, it works sometimes but that doesn't seem like a good idea!

 

Instead of WD-40 trying first cleaning all of the moving parts with carb cleaner to clear away the dirt and grease, then once dry, coat all moving parts with silicone spray.

 

I had the same in my very old volvo. It was moisture in lock mechanism. I used a blower with warm air on door for a long time and problem was solved.

After that when I had more time I take out trim from door, cleaned mechanism and use new grease.

 

Don't forget WD40 has water in it. I always use PB Blaster Lube or a dry graphite lube. I had the same problem on my old Expedition too, until I quit using WD40