No air either hot or AC is coming out through the front or floor vents, but will only come out of the front defrost vent on the top forward of the dash.
It's a 2001 F150.

Does anyone know what might cause this?

Thanks so much!!

 

More then likely its your blend door or linkage on your heater box. I`m not familiar with the layout of the 2001 f150 but you sould be able to see the cables and linkages goin to the levers on the heater box under the dash. Have a look and if you can try moving them with your hand and see if the air flow changes. If its the door itself you will be into some work to get to it. You should be able to hear the door moving when you change the position on the dial from floor to face to windshield. Do you hear anything at all?

 

No, this is not a blend door problem! This is a vacuum leak problem and the air is defaulting to the defrost by design. A blend door problem would be indicated by the lack of control of temperature.
Check the vacuum line at the firewall on the passenger side behind the battery for breaks or cracks.

 

Had the same issue with a 1978 Ford fairmont futura. (No air from panel or floor vents, only defrost)

Found a vaccum hose not connected in engine compartment at firewall.