The blower motor resistor module is ruined and need replacement. It can be found from below and behind the glove box. Tt may be hard to see, follow the wires to connection that is mounted to the blower unit by two small hex-head screws. I removed the glove box so I could access it easier than trying to go from the bottom in a confined position.
the blower resistor controls the fan speeds
2004 Chevy express van, heater has no high speed, at one time the blower stayed on even when the ignoton was turned off therefor draining the battery,
If you have a 2 stage furnace (hi fire & lo fire) then the blower will run on lo speed in lo fire & hi speed in hi fire and in AC mode. If it`s a single stage furnace lo speed for heat and hi speed for cooling.
the speed at which work is done is velocity
work
either the blower control switch or the blower motoer needs replacing
I would suspect the blower motor speed resister is shot.
If you are referring to the blower motor only working on the high speed setting, then the problem is the blower motor Resistor Pack is defective. Replace it. It can be the blower motor speed control switch but not likely.
If the blower works on slower speeds only I would suspect the high speed blower motor relay (under the hood in the power distribution box) or the switch itself. The blower speed resistor is working if all the slower speeds work.
replace the blower motor resistor
If the one speed it works on is high speed, it is the blower motor speed resisitor that is bad.
The blower motor speed resister is burned out. This part is available from your local auto parts store.
Probably a bad blower motor resistor
The blower motor speed resistor is burned out. The high speed function bypasses the resistor. The speed resistor is generally located near to the blower motor.
blower motor resistor is burned out
If you only have high speed I would blame the blower speed resister. If you have no speeds and the fuse is good, I would blame the blower motor or the switch.
try replacing the blower motor resistor