There is a little motor, a fan, and coils. These coils heat up with the motor causing heat that heat is then pushed by the fan into the direction you want it.
something has to be blocking the vents.
Loose connection or blower motor? Bad blower motor? Loose fuse? Bad fan switch?
Heater core
when a blower motor only works on high, it is usually due to faulty resistor board. Should be located in the duct near the blower. the purpose of this is so the air blown by the blower keeps the resistors cool.
it is due to the difference in the air intake and impeller construction.
You most likely have a bad blower resister..........
possibly a blower motor. or switch
the resistor for the blower motor is busted blower works on high with out it
Figured it out...the blend door actuator was not fully seated when I replaced it.
more than likely will be the blower motor resistor has went out.
Either you have a faulty blower fan in the front, or power isn't getting to it.
My guess would be the blower motor speed resistor is burned out.