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The Lennox 83L91 (R40154B016) is a genuine OEM control-limit switch that provides critical over-temperature protection for your Lennox furnace, cutting power to the burner when internal temperatures rise beyond safe levels. A malfunctioning limit switch can cause symptoms such as short-cycling, no heat, or a blower fan that runs constantly without producing warm air. Replace it with this exact OEM Lennox component for assured fit, function, and long-term reliability in compatible furnace models.