Bad High-Limit Switch on a Furnace

Power It Down

• The first step to checking out the high-limit switch to your furnace is to power the entire furnace down. You should turn the furnace off at the central controls as well as switch off the breaker that supplies the furnace circuit with power. Working on a furnace or any other electrical appliance with the power connected could potentially lead to shocks or electrocution. With a gas furnace, you should shut off the supply of gas to the furnace as well prior to working on it.

Testing Resistance

• Testing your high-limit switch for resistance will tell you whether it is functioning properly or not. This is a reliable test that is easy to perform if you have the right equipment. Using an ohmmeter, a small device that will allow you to gauge the resistance in the switch, you’ll be able to confirm whether the switch is working properly. Assuming the furnace is below the set point temperature, you can disconnect the wires from the high-limit switch and clamp on the ohmmeter probes in their place to see if there is a problem. A properly working switch will read zero on the device. Otherwise it needs replacement.

Run Another Cycle

• If your switch is bad, purchase and install a new one and reconnect the wires to get the furnace working again. Turn the power and gas back on and run a heating cycle. Make sure the furnace has proper airflow and is producing heat normally.

Other Potential Problems

• If your test for high-limit-switch problems comes up negative, then you probably have other factors that are causing the system to shut down or overheat. In these instances, the high-limit switch is just doing its job by preventing overheating. The gas pressure to your furnace may be too high, causing excessive heating, which trips the switch. You may also have clogged air filters that need to be changed to increase airflow through the system. If you still have problems, you should check for obstructions in the system ducts or other areas where airflow may be choked off by debris. Beyond these steps, you should contact a professional for a service call.