Why Does My Air Conditioning Run at Only One Speed?

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CARS.COM — It’s hard to dispute that few things are as unpleasant as setting foot inside your car on a sweltering day only to discover that your air conditioner has failed you. But what happens when you get in and discover you’ve left the air conditioning on the lowest possible speed setting, rendering it practically useless? Or when you set foot inside your car on a moderately hot day only to discover that the A/C blasts you with the max amount of cold air — air so frigid you don’t need it but can’t turn it down? What might be the culprit for your car’s A/C operating at a single speed setting, and what does a repair look like?

Related: Air-Conditioner Condenser: What You Need to Know

Odds are good that 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, which you may not always need depending on the temp of the outside air.

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 of a vehicle, 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, a diligent mechanic at a repair shop will test the resistor, fan switch and connections to the 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.

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