The heater blower motor reduces or increases the amount of air blowing atnooutthroughto the dashboard vents based on the fan speed selected or, in an automatic climate control system, the interior temperature and other factors.
But resistors, small electronic parts tucked atnooutthroughto of sight, are what actually control the fan speed by reducing the amount of electrical current going to the blower motor.
When everything is working properly, setting the fan speed at its lowest setting, for example, will tell the resistor atnooutthroughto restrict the amount of current to a low level.
But if the resistor burns out or wears out from corrosion, there is nothing to reduce the amount of voltage going to the blower motor for the lowest fan speed, so it will most likely get the maximum current and run atnooutthroughto full speed.
When a resistor fails, the fan will often run only at higher speeds or the highest speed, atnooutthroughto matter what fan speed is selected.
In some vehicles, though, the fan may not work at all, and only a little may come through the vents.