The air flow vent is stuck on defrost mode. when i switch to a/c or face/floor no air is directed out. the fan seems to be working fine

Does the blower only work on certain speeds? Blower motor resistor. If it dosent work at all it’s probley the blower motor itself.

Are ALL HVAC fan speeds operational when requested ??
With fan set on Hi,switch modes....Does the air flow come out the appropriate vents w/requested (Defrost,Dash center,Floor ????)

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

The blend is not working.

What year is your Envoy ? Probably high speed relay .

The most common problem is a stuck or faulty blend door under the dash.

I the short answer is yes. The truck has a flow-thru vent feature as does most GM products. When the fan switch is off the outside air would flow into the truck.
I thought you could still direct the air by way of the control panel, but by the wiring diagram it looks like everything is disabled when the fan switch is in the off position.

Blend doors not working. They are controlled electronically by small motors. Could also be something fell down in a vent and is blocking it from it's full range of motion.

Does your vehicle have manual or automatic climate control ? You system is stuck in floor an defrost mode ?

Item Part Number Description 1 18A318 Vacuum control motor-floor/defrost door 2 - Defrost air flow 3 18A478 Floor/defrost door 4 18476 Heater core 5 19860 Evaporator core 6 19805 Blower motor 7 - Outside air inlet 8 19A813 Air inlet door 9 18A318 Vacuum control motor-air inlet door 10 - Recirculated air inlet 11 18B545 Temperature blend door 12 18A559 Panel door 13 - Panel air flow 14 18A318 Vacuum control motor-panel door 15 - Floor air flow 16 - Vacuum from the engine intake manifold 17 19A563 A/C vacuum check valve 18 19A566 Vacuum reservoir tank and bracket 19 19980 EATC control
Pull the control panel an check for vacuum

Item Part Number Description 1 White Air inlet door 2 Yellow Floor/defrost door 3 Red Floor door 4 Blue Panel door 5 Black Source vacuum 6 - Not used


VACUUM APPLICATIONS CHART-ELECTRONIC AUTOMATIC TEMPERATURE CONTROL Switch Port Vacuum Harness Hose Color Func- tion Manual Override Selector Buttons OFF MAX A/C NORM A/C VENT FLOOR FLOOR/ DEF DEF 1 White Air inlet 2 Yellow Floor/ panel (partial) 3 Red Full floor 4 Blue Panel 5 Black
Source

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.

the deflector flap that move to direct air flow is stuck under the dash.