If you have a meter and are comfortable working with it consider exposing the wires behind the selection knob. See if you have power into the switch and power out of the switch to the low speed setting. If there is power in but no power out of the switch, the switch is to blame If you have power in and out of the switch, one needs to get the wiring diagram and trace the power until it is lost. At that point is the failing component.
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