When your gas furnace’s limit switch fails, you will need to replace it. First, turn off the electricity to the furnace at your breaker box. Then, remove the furnace panel and completely detach the wires connected to the limit switch. Once you’ve removed all of these parts, mount the new limit switch with a mounting bracket (also called a saddle) and reattach the wires. Finally, put the furnace panel back on and turn the electricity back on.