In my 2005 Nissan Sentra when I turn the heat on i turn the controlled to put it on defrost floor or vent it only works on vent what could that be

flapper sticking or sometimes the gaskets that go around the seams come out and block the flappers

The defroster works from the same fan as the vents, so the most probable cause is that the ducts from the fan housing to the vents have come undone. fixing it involves getting way underneath there to see if you can locate the loose ducting. You might have to remove the glove box and / or the instrument cluster to see what is going on back there.

MODE ACTUATOR IS INOPERATIVE. LOCATED BEHIND RADIO ON THE DUCTWORK

Check for a vacuum leak in the control panel

HERE'S A LINK TO YOUR SET UP,SOMETHING COULD BE BROKE,NOT LETTING YOU TO ADJUST WITH THE SWITCH,THERE ARE CABLES ATTACHED TO THE LEVERS THAT MAKE THEM TURN WHEN YOU TURN KNOB,MAY BE LOOSE,BENT OR BROKE,YOU WILL HAVE TO REMOVE THINGS AND SEE.

check for a broken /loose vacuum line on the engine . Trace it back to the fire wall and it will most likely be the culprit/

The flow switch it change's howthe air flows get a motor manual at auto parts stir about 25 bucs it will tell u what up

This is usually caused by the selector switch being bad on the car. You will need to change the switch for it to work again.

vent door is stuck open

a/c is active when in defrost mode to remove moisture from air. what controls the movement from vent to heat/ or defrost. a lever or switch.