2008 g5 defrost air not blowing air

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: PROBLEM WHEN SELECTING THE DIRECTION FOR THE AIR

Sounds like you have a supply vacuum problem. When you lose supply vacuum to the control head it defaults to defrost.

SOURCE: All Air Conditioning blows our the defrost vents

Some thing you could look at would be this. The control is usually fed by a vacuum line or small hose thats is connected to your intake manifold.
With a cold engine,look for a skinny vacuum hose that runs from the intake toward the fire wall . It should be going inside the cab and connecting to the A/C vent control.Some times the hose becomes disconnected or splits or cracks due to heat under the hood.
Thats usually the trouble.
I hope this helps you.

SOURCE: Blower air just through defroster vents, not front vents

The problem is that the plastic lines that the air flows through get frail and cracks and breaks.What happens is when the hose breaks you loose vacuum and the system reverts to defrost mode.You should be able to see it from under the hood on the passengers side.You will have to find the area that is broken and then go to the parts store and get some rubber hose that will fit over the plastic hose.Take a piece of the plastic hose with you so you can get the right size hose to fit over it.This just happened to my 97 Astro van.I found the broken hose and repaired it with the rubber line and is working normal now.Hope this helps.Let me know what you find out.Good luck.

SOURCE: when i turn the air on it doesn't flow from the front vents.

Sounds like the switch or actuator is not operating. Some vehicles have mechanical operators, and some have vacuum actuators. If a vacuum line is loose, it will not actuate.