[How it works]—The limit switch monitors the temperature of the air passing over the heat exchanger,And will shut off the furnace if the system overheats.Temperature Limit: 203°F. [Why we need it]—Any restriction of the air flow might cause the heat exchanger to overheat,This is why it is essential to replace the furnace filter every three to six months.If the blower fan continues to turn or if the blower fan cycles on and off,then limit which might faulty. .