By Rick Popely, Cars.com
The fan switch may be at fault, but a more likely reason is that a blower motor resistor that controls the voltage going to the blower has packed it in, resulting in only one speed remaining operable, usually the highest speed.
When a blower resistor fails, the highest fan speed usually is the only one that still works because it essentially bypasses the resistor and receives the maximum amount of voltage. When the resistor is working, it reduces the amount of voltage going to the lower fan speeds so the fan runs slower.
Blower resistors are small parts that can become corroded or burnt out, and a new one often costs less than $50. Because they're usually under the dashboard on the passenger's side or behind the glove box, near the blower motor, they can be hard to reach if and when they need to be replaced.
Though having only one fan speed usually indicates a bad blower resistor, diligent mechanics will test the resistor, fan switch and connections to the blower motor to make sure nothing else is wrong before they start replacing parts. If none of the fan speeds work, then checking the fuse or fuses for the heating, vent and air-conditioning systems is a logical first step.
