2005 gmc envoy manual no air flow from front dash vents defroster and floor vents are fine

Sounds like the diverter door is bound up causing the motor to bog down under load. You'll need a manual for that job. It's a pain in the rear to get to.

That's the mode door , not blend door . electric actuator motor is bad . replace it .

What year is your Envoy ? Probably high speed relay .

Under the dash you have the vacuum motors or servos or
electric door motors, for the hvac switching of air flow

So a vacuum leak or bad motor, whichever you have or binding
doors,also there may be linkage to some of them that gets into
trouble.

Hard to work or see things under the dash

If you have any electronics with an auto heat & a/c system then you
will need manuals to diagnose

I have NO vents blowing on the dash , shifted from floor to defrost no trouble??? Help...AC or Heat

Is it a new vehicle? I had a new vehicle that had the same problem. Answer was that the holes weren't punched out of the vent.

the resister block needs changing... oddly enough i'm unsure of what fan you are saying is not working.... if it's the front.. the resister block is located on the duct work under the dash for the air... near the fan... you will see a connector going into the side of the duct work. two screws and the block comes out... should be fairly obvious for the rear one but it will be near the fan.

Try replacing the cabin air filter.

I got the same problem. I was told it was the AC control module that goes bad and it kills the ability to blow air out the AC dash vents. My Envoy is a piece of junk, electrically speaking.