Heater Core is stopped up with debris not allowing for smooth air flow through it. Or something stuck in the back of the fan assembly not allowing the fan to be able to pull air through. Found one of those plastic grocery bags covering the back of the fan in my 1997 Suburban
Good luck.
It should have nothing to do with the vents itself. More than likely it's the blower motor that's not working.
When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.
Air flow is controlled by plastic doors which are controlled by vacume motors. Could be they're stuck, or a vacume leak.
The year, make and model would help but many vehicles use vacuum to operate the direction of air flow through the floor vents, dash vents and defrost vents. If the vacuum supply is cut off for whatever reason the system defaults to defrost only. So if you only have the defrost mode working chances are the vacuum supply is cut off. Check the HVAC vacuum supply line under the hood for damage. If that system seems to be working but you have insufficient air flow through all of the vents, check/replace the cabin filter if applicable.
No air flow? Check for vacuum leak either under dash or at engine compartment Maybe bad vacuum diaphragm under dash Maybe bad selector control
either faulty switch on dash, acutators inside the dash, or broken doors or plastic inside of dash.... common problem on the 2002-2004 or 2005 there was a total dash replacement kit it change out the broken plastic pieces inside the dash
I call it the HVAC (Heating, Ventilation and Air Conditioning) box.
a person satnding in the way of the vents
You need to look under your dash and make sure all the heater vent tubing is still connected and if so there is a flap located about the center of the dash that transfers the flow of air from the dash to the vents it could be hung in one position,hope this helps,Ricky.If that doesnt work then you need a new blower motor
Replace the cabin filter. remove panel under dash on pass side.
Yes
You will need to check the vacuum for the air door operations first and then check to make sure that the doors are working correctly in the dash...........