Yesterday I started my truck set the selector to defrost and it stayed blowing out the dash vents. Tried shutting off the fan turn it back on same thing. Shut the truck off restarted it same thing. It was around 5 degrees F so I pulled it into my heated garage for the night thinking one of the doors in the box had broken. This morning I fired it up just for the fun of it to see if I could get it to defrost and magically it works as it should. Any ideas why it started to work again? Truck is an 03 2500

 

OK, here is a theory (may be thinking out my backside though, I'm not a professional HVAC guy).

As you run your system on Defrost, the air conditioner compressor is put in use. Of course, you have your heater turned up to high so you won't get cold air, but the evaporator pulls condensed moisture from the air and it normally drains out a tube to under the truck. In the summer, you would notice a puddle under your truck while running the A/C.

So, back to the theory . . . if the drain tube is clogged (it happens) the water that would normally drain would build up. When you shut down for the night, the water freezes because you parked it where it is colder than a witch's . . . oh, nevermind. The door for the defrost in the HVAC unit is pretty low so if the water got down there before it froze, it probably froze the door in its last position. You warmed it up in the garage, melted the ice, and thus the door works again.

Somewhere on this site, there is a procedure to clean out that drain. I would clean that out first.
Good luck, back to morning brew . . .