A/c heat, defrost control is not working ! its on defrost and wont switch

SOURCE: There is no air flow

You need part GM Part number 89018675 or ACDelco 15-73596. The Mode Selector Actuator. What happens is the part is a CHEAP part POORLY designed and when it goes to turn the flaps inside the HVAC unit its the plastic gear slips instead of turning the flap!!!

Just to forward you you might want a deal to install this art for you unless your good at fixing your own vehicle, because you need SMALL sockets and a lot of patience to get this part in!!!

The part is ALMOST directly above the steering shaft for your steering wheel, so theres a lot you got to remove before you will even see them!!!

I only recommend you to have a dealer put this part in for you, unless you have your own service manual!!

Because there is 5 of these darn things and if you pick the wrong one you will have to do it all over again!!!

3 on the Drivers side and 2 on the Passengers side!!

But replacing this part SHOULD solve all your problems!! AS it did for me and i was having the same issues as you have described!!

SOURCE: Heat wont blow through heater or defroster vents

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

SOURCE: AIR FLOW STUCK IN DEFORST VENT WILL NOT SWITCH TO FLOOR OR AC VEN

I found a broken vacuum hose on top of the intake which runs the controls

SOURCE: A/C only blows through defrost vents

It sounds like the blend door actuator which diverts air from defrost to vent to the floor.
The bad news is it's a bear to get at. It's located behind the glove box.

SOURCE: 2003 Trailblazer No air from front face vents

first check to see if you have power going to the actuators when you switch the switch to it location if no power replace controller if you have power and no movement from actuators replace actuator. if actuators move remove hvac box and check for broken blend doors.