Well, there's more than one door in an HVAC system, and there are three possible means of actuation of those doors. You gave no information which narrows any of this down - not even a vehicle make and model, from which such information could be found out. Your question needs to include these things before it can be answered.
If it is broken. It could be a faulty temperature blend door actuator.
Start with: Possible control, blend door or blend door actuator, low pressure switch, leak in system.
The temp blend door. It is electrically controlled by an actuator. It is located behind the dash assembly.
You have a faulty temperature blend door actuator located directly behind the radio. Remove the radio and remove the blend door actuator held to the heater case with three 8mm screws, and replace it.
Where is the blend door actuator on a 1998 Sonoma
It depends on the year and wether you have digital or manual air
replace temperature door actuator.
There are actuators all over the HVAC housing.
Clicking sound in grand Cherokee can be due to a faultyÊ actuator, or a cracked door. Actuator may causeÊ a clickingÊ sound when one of its plastic gears is broken.
The air door actuator on a 1998 GMC Jimmy is located in the dash. It is located behind the glove box.
where is the defrost/floor door actuator on 2002 chevy trailblazer
It controls which vent the air comes out of.