What Causes the Air Conditioning Fan to Slow Down?

By Rick Popely, Cars.com

If your fan no longer runs at the high speeds it used to, then it's probably worn out from years of use or has other issues that limit airflow. But if the fan still makes lots of noise at higher speeds and only a small volume of cool air is coming through the vents, there's a good chance that a plastic "blend door" is broken or stuck in the wrong position.

Blend doors open and close depending on which mode the climate system is in, such as air conditioning or heat, to control where the air goes and the air's temperature. When the air conditioning is on, electronically controlled blend doors are supposed to close off heat so that only cool air goes into the interior. If a blend door for cool air is open only part way, the fan might generate a mighty roar, but only a small amount of air will make its way into the interior.

Actuators that control when the doors open and close can fail, and in other cases the doors can stick or break. Some of these are easy to find and replace, but most involve professional diagnosis to pinpoint and the removal of the dashboard to replace them. With automatic temperature control, sensors that tell an electronic control unit how to manage air flow also can fail, and the result might be too little cool air being directed to the interior.

Other possible causes of low air volume (if the fan isn't the culprit) is that the cabin air filter is clogged with dirt and debris, restricting the amount of fresh air going into the air-conditioning system, or that debris, such as a plastic bag or wrapper, is blocking the flow of air.