The blower motor resistor block has burned out , this happens 9 out of 10 times from the blower motor becoming old and worn, when the bushings in the blower motors wear the motors actually draw more current to turn as the amperage usage goes up the resistor block actually burns out saving the electrical wires going to the blower motor keeping them from burning, to repair this both need to be replaced
This sounds like a bad blower motor resistor board that has gone bad. they are designed to work only on high if they go bad so you will still have the blower to defrost the windows.
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The blower motor resistor block has burned out , this happens 9 out of 10 times from the blower motor becoming old and worn, when the bushings in the blower motors wear the motors actually draw more current to turn as the amperage usage goes up the resistor block actually burns out saving the electrical wires going to the blower motor keeping them from burning, to repair this both need to be replaced
This sounds like a bad blower motor resistor board that has gone bad. they are designed to work only on high if they go bad so you will still have the blower to defrost the windows.