The blower control module has gone bad. Carquest has a new one for $20-$25 dollars.
YOUR BLOWER RESISTER IS BAD
Check the blower motor resistor
Usually the blower motor resister has gone bad.
The most likely problem is the blower resistor located behind the glove box door.
That is usually a failed blower resistor.
When the fan is on high it does not run through the blower motor resistor. The low speeds don't work because the blower motor resistor is bad.
blower motor resistor needs to be replaced
The blower resistor has failed.The blower resistor has failed.
blower motor resistor
The blower motor resistor has failed.The blower motor resistor has failed.
If you mean the blower only goes in the high mode check the blower resistor located in the plenum under the wiper module.
The most likely problem with the heater blower on your 2003 Sable is the blower resister. The resister can go bad and cause you to only have one speed.