a/c and heater works only on high in 2000 Pontiac Grand Prix

For some reason my a/c and heater will only work if i turn it to high. This started all of a sudden. I will take all of the advise i can get.
My car is a 2000 Pontiac Grand Prix. Thank you!!
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Your resistor module is bad. It's a group of several resistors that drops the voltage to the fan to change speeds.
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