Take a meat thermometer (electric or not) and place it in the vent so it is not touching the sides. Turn the AC as cold as possible, and set it the blower on low. It should read 40 degrees within several minutes (or close to 40 degrees). If it is, then it's working properly. If you turn the blower up, you'll notice that the temperature will go up. For example, I set the blower on low, it read 40 degrees. Then on high, it read 50 degrees.