What does the blower resistor do in a 2006 Chevy Express?

In a 2006 Chevy Express (and most vehicles with multiple blower fan speeds), the blower motor resistor controls the speed of the blower fan. It does this by regulating the voltage going to the blower motor.

The resistor is essentially a variable resistor; it offers different levels of resistance to the electrical current. Higher resistance means less current flows to the motor, resulting in a slower fan speed. Lower resistance allows more current to flow, resulting in a faster fan speed. Each fan speed setting on your control panel selects a different resistance setting within the blower motor resistor. If the resistor fails, you may lose some or all of your fan speeds, often only having the highest speed (or none at all) functioning.