A high-limit switch is one of the most important safety devices in your furnace. It basically keeps your furnace from catching fire and burning down your home! A high-limit switch works by shutting down the burners and turning on the blower motor if the heat exchanger reaches a pre-determined temperature. The one pictured above is 180 degrees with a 40-degree differential. This means that once that temperature is exceeded, the limit switch opens the circuit and shuts down the burners. This particular one shown above is an auto-reset limit. This means that once it cools down, it will close the circuit and allow the burners to operate normally again.
The most common reason for one of these to fail is repeated tripping of the limit from a dirty or blocked air filter. Another reason would be a clogged evaporator coil or a cracked heat exchanger.
You can test this limit switch without a meter by simply removing it and holding a lighter under the metal disk. (shown in the photo above) If after a few seconds under the flame you hear a pop, then you let it cool down for a few more seconds and hear another pop, it is working properly. If not, you need to replace it. If you do have a multi-meter, all you have to do is set it to ohms and put your probes on each connector, if you read ohms, it is good, if not, it needs to be replaced. It is very important that you get a replacement with the exact same temperature setting. You can use the Fixmyfurnace.com chat option and ask us to double-check or ask us any questions at any time. We are happy to help make sure that you get the correct part every time!