Air flow only works downstream. Why won't come out the dash?

There are upstream and downstream O2 sensors for your exhuast. Are you referring to where the air blows for your AC/heat?

If your talking about only having air flow say at your feet, but it wont come out the dash head vents, or the defroster vents, theres no sensors. your mode doors arent moving, and its more than likely due to a vacuum leak.