I have a 2003 Hyundai Accent GSI. The main defrost doesnt work well. The fan seems to run fine, it blows out the dash vents just fine, but barely anything blows out the defrost vents. There is plenty of heat from the dash vents. Any suggestions?

A vent flap actuator probably needs to be replaced. It's what moves your vent flaps from heat to A/C or to plain "vent air" when you change settings.

The discussion in the link includes vent flap actuators. Unfortunately, they also include the infamous no low speed fan problem too.

Cannot get any air from dash vents while defrost and floor vents work fine...

Your particular actuator will be a different one than they found.
They're all a absolute pain to get at because they can be well buried in the dash. I would start looking under the dash on the passenger side, that's where most of the air control is. Cannot get any air from dash vents while defrost and floor vents work fine...

seems that the evaporator is freezing and does not allow the air to passthrough

could be the vent door actuator not opening all the way . under the dash there is the blend door actuator and the vent actuator that are controlled by vacuum

Mode door actuator . A electric motor that controls a door that directs air flow .

usually a faulty blend door under the dash

there is a thin plastic air line running from engine compartment through fire wall into the dash check to see if it is cracked in the engine department over the years they get hard a crack letting some air get through but not enough to change vent settings

Had the problem been solved after you replaced the damper door switch and had the gas checked in the ac system??

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ak90 programmer

The a/c system is designed to cycle on and off when in the defrost mode. This helps pull moisture out of the air to help clear your windows

Check to see if the duct hose to the defroster vents has come loose. If air is blowing out under the dash when you have the control set to defrost, then this is likely.

have the water pump checked out, i had a van that did the same thing and founf i had a bad water pump it fixed the problem.also check the blower motor to make sure its working right by tapping on it while you have the car running. the car will survive dont worry its going to be one of those two things im pretty sure.