No heat in the front. What's a heater blend door?

I have a 2000 Expedition. Heat in the rear is fine (separate heater), but about zero up front. Had a bunch of guys tell me it’s a common thing for the heater blend door to fail. What does it do? Where is it located? And how do I go about diagnosing my problem?

The blend doors hide under the dash inside the chamber between the dashboard and the firewall.
All of the air that comes out of everywhere is blown by just one fan. The blend doors open , close, and move to direct the air to where you set the controls, ie; defrost , floor, front vents , or any combo.
Some doors are moved by little electric motors housed in small plastic shells about 3x3x1 mounted in various locations up under the dash. Inside the plastic shells with the electric motor are many little plastic gears much like a clock works. Those little gears can strip out and the door won’t move when you ask it to.

  • Other doors are operated by vacuum ( suction ) motors. A vacuum leak or loss of suction will default the door to closed position.

A tech need to look this over to test many variables.
Vacuum leak.
debris blocking the doors.

It gets even more complicated if you have ATC Automatic Temperature Control, where you set a temperature like 70 instead of just the knob for warmer or cooler.

The most commonly sold item for that type of issue is the electric motor, Ford # 2L3Z-19E616-BA. You’ll see it when looking up at the under side of the chamber near the middle.