"The problem is mold on the evaporator face that grows over time. The thing is a couple years ago the government cracked down on certain chemicals everything we use now is water based and doesn't work so well. The same thing with chrome plating they are all peeling off where as years ago they never had that problem. The other thing that is affected with this poor quality water based chemicals is the anti microbial coating they use on the evap it just doesn't do its job anymore. I have noticed it on the Fusions big time and recently had to change one out what a PITA. Since then ther has been a revised evap to prevent this on the Fusions and Ford Edge but nothing coming down as far as a revised one for the F150. When you get your new one turn off the a/c a block or two from your destination and run just outside air full blast no recirc that will dry out the evap so it is not soaked and sitting in a dark warm place mold like to grow. I have done that for year on all my fords never a problem. Now if you are just running into the store quick obviously you don't need to do that but lets say parking it at work or home overnight."