Dear Tom and Ray:
My car is a 2005 Hyundai Sonata. The fan just started blowing air full blast whether the switch is set to on, off, high or low. I pressed all the buttons and turned my car off and on several times. The heat and air conditioning still work. Help! — Lindsey
Tom: You need a new resistor, Lindsey. Or you need to resist the temptation to use your car.
Ray: A resistor is a component that creates electrical resistance. If you think of electricity being like water running through a hose, a resistor is something that constricts the hose, making it narrower and allowing less water to pass through.
Tom: The speed of your fan is controlled with resistors. Let's say your fan switch has four settings: off, high and two middle positions.
Ray: When the fan is set on full blast, there's no resistor involved. A full 12 volts of electricity is sent to the fan motor.
Tom: If you turn down the fan switch one notch, one resistor is then engaged. That reduces the amount of electricity sent to the fan motor, which reduces the fan speed. You switch the fan to the next-lower speed, and yet another resistor is engaged. And so on, until you have the fan switch set to Off, and there's no electricity getting to the fan.
Ray: My guess is that your resistors are fried, so you're always getting a full 12 volts to the fan, and it's always running at high speed.
Tom: There's a little box under the dashboard that's called, surprisingly enough, the fan motor resistor. It's actually a set of resistors, but the part is referred to as "the resistor."
Ray: You need a new one. Or a bunch of paperweights and magnets to keep things from blowing around in your passenger compartment.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)