May be a loose connection or an open resistor? maybe a loose fuse in the fuse holder? Bad fan switch?
Check the resistance on the blower motor.
Common causes, blower motor resistor, blower motor, wiring or controls
usually on or near the top of the blower motor behind glove box area
directly above where the passenger's feet are.
usually what causes that is the blower motor resistor. Get a volt meter and test it according to it's values, 90 percent of the time it's the RESISTOR.
Check to see if you are getting power and ground to the blower motor - if so blower motor is bad. If no power possibly fan switch or selector switch Also check the blower motor resistor, found in heater plenum.
Probably the blower motor, wiring to it or control switch
Usually the blower motor resister has gone bad.
The blower motor speed resistor is probably burned out.
The blower fan / motor is mounted behind the glovebox. Remove the glovebox for clear access to the 3 or 4 mounting screws.
Defective Blower Motor Resistor Pack.
Blown fuse, defective blower motor, defective wiring, defective blower motor resistor pack, or in rare instances a defective blower motor speed control.