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My heat and air only blows from my defrost vents and none other. Heat and air both work as they should temperature wise and it blows pretty decent from the defrost, just can’t get it blowing from anywhere else no matter what setting I have the switch on.
 

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Ford's HVAC is famous for eating the little plastic motors that move the air doors which they call "blend" doors. I've only ever done them on Towncars which are a royal pain to do but I've done plenty of them. If they are actually accessible under the dash then that's a huge bonus. On my Towncars there's an EATC self test you can run which will sometimes resurrect a dead system. The self test cycles all the motors and sometimes that's enough to start them working again. It usually doesn't last but its always worth a try. I can't find anything on line to tell me how to do a self test on a Superduty EATC but I find it hard to believe you can't because its been on the cars since at least my '97 Towncar. I just went and tried what works on the Towncars on my '05 Powersmoke and it didn't work but Google's your friend. Maybe you can find something. If you can run a self test it will sometimes throw a code to tell you what it thinks is wrong. I seem to recall that you can initiate a self test from Forscan as well but I don't care enough to drag my laptop out to the truck to test that theory. Good luck - the motors are cheap but you need the right one because they have different actuator arms and mounting brackets depending on application.
 

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Sounds like a vacuum leak.
 

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More than likely it's a vacuum problem. The fail safe or default for the HVAC is to defrost. That should have a small electric vacuum pump mounted on the right side of the engine compartment near the fender. The pump supplies vacuum for the HVAC controls and the 4X4 hubs. It could be the pump has a power supply or ground problem or it could have a leak in the 4X4 hub system, the vacuum reservoir or the hoses to the HVAC controls. The hub system is usually where the problem is. You can start by plugging the hose to the hubs off to see if that corrects the problem. Just a FYI, a blend door refers to the door that controls temperature only. There is also a mode door, and a recirculation door.
 

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The OP doesn't seem to be all that excited about it. Hasn't logged on since the day of his original post.
 

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hold on a sec, i logged on to troubleshoot this specific issue so don't bail!!!. unlike the o.p., my hvac system intermittently enters "failsafe" mode and only vents at the defrost vents. sometimes it will revert to the dash vents, even while underway. am i to understand this is a leak in the vacuum system? is there a hack, or an easy way to test the system? how hard is to replace the pump? truck is 4x2
 

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check the vacuum hose to the heater valve
 

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