I have an '07 ford explorer sport trac. A/C only blows only from windshield vent. Think the knob doesn't work anymore. What's wrong with it?

I have an '07 ford explorer sport trac. A/C only blows only from windshield vent. Think the knob doesn't work anymore. What's wrong with it? - 1

Yes

The knob doesn't work anymore

Probably a bad door actuator.

That's a typical problem caused by a vacuum leak in the HVAC control system which makes the system actuators revert to a default position of blowing air from the upper vents. Check the vacuum lines under the hood for leaks… Broken, cracked, loose lines. A common problem area for lines is behind the battery or firewall or a bad check valve near the left rear engine cover. Check the vacuum reserve tank for leaks too… Looks like a big plastic ball or oblong plastic part behind the glove box. If the lines /tank are all good and you get vacuum up to the firewall, then possibly the control head/control switch is bad. Listen for any slight hissing noises near lines or the control head with engine running that indicate a vacuum leak.

https://www.ford-trucks.com/forums/882747-a-c-moves-from-panel-to-defrost-and-back-under-acceleration.html

https://www.justanswer.com/ford/3ninx-2008-explorer-ac-not-cold-80-deg-vent.html

It sounds like it is broken.