Heater stuck on defrost

What make an model an year vehicle do you have ? The air blows out of the floor vents an center vents but not defrost ? Possible mode actuator , stuck mode door ! There is a door inside the heater / A/C assembly that directs the air to the desired position . These doors can be moved by a vacuum operated actuator or an electric motor . Sometimes thing's put on the dash end up inside the heater box , fall through defroster vent ! Pens , pencils etc.. Depends on the make vehicle an system , manual , or automatic climate control . Some of the newer vehicles the HVAC system as it is called , the control panel is a electronic control module an can store DTC'S - diagnostic trouble codes !

Usually, it's a door motor on the heater case that's stuck/broken. Is this an automatic HVAC system or a manual system?

go by liquid coolant and check ur heat and ac fan it might be stuck or broken

The heater blend door actuator (motor) is faulty.Replace actuator

does it blow through the defrost vent only when you switch to vent/floor or does it blow through defrost and vent/floor? is it a switch that is just electric or a manual lever that you slide? the manual type will have that problem if the diverter or the linkage behind the lever(in the dash behind the panel......may be able to access underneath) becomes broken or disconnected so that the air is diverted to defrost regardless where you move it.if you can get to it, you will see what is broken or maybe a fastenerr fell off.

either a vacumn pipe is broken or the ends split so its not passing a vacumn to open the flaps,where they are and how to get to them i cant help because i have never worked on one as they are not here in spain.check the engine end first though .Look for some small rubber pipes running from the inlet manifold

vent door is stuck open

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.

vacume hose off or cracked it wont change to floor heat ooorrr field mice have made them selves a home in your heater box

Had the same problem. believe it or not i changed the gas cap and got heat all around. something about pressure.