I have a 2001 silverado Z71 with the console option,all vents work as should except very little to no air coming from the floor vents on either passenger side or drivers side ,the heat and a/c works as should and all upper vents seem ok ,is it a blend door ? or the switch on the dash? any help would be appretiated Thankyou .

SOURCE: Blend door not working.

check the vacuum supply and vac lines from the control head for a leak

SOURCE: 2005 chevrolet silverado z71 1500

I had this problem in my pontiac grand prix. it was the dual zone door actuator.( I can't remember if that was the technical name) basically there is a door underneath and behind the dash, depending on how much cold or hot air you want there is an actuator that opens or closes the door.

SOURCE: A/C Heater problems

the main issue with this is either the thermostat or the climate control switch I do not know if your climate control switch is electronic or manual either way I would start out with the cheapest and go with the thermostat first good luck!

SOURCE: Defroster & floor blowers don't work but front facing vents do

There are what they call mode door's these days they have a electrical motor that turnes the door to different posistions...sounds like yours is become broken. It would depend on your expertise in the ac & heating system area to replace the part. Sometimes easy and most of the times the dash has to be removed.

SOURCE: airflow problem through ac vents

Your selector panel is bad, ( temp, blower, ect.) all one unit, my 95 did same thing chg. that unit and it fix it.