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The Lennox 42J0601 (42J06) Fixed 120 Limit Switch is an OEM safety control designed to protect your Lennox furnace by automatically interrupting burner operation when internal temperatures exceed the fixed 120°F threshold, safeguarding the heat exchanger and other critical components. Symptoms of a faulty limit switch include the furnace shutting down unexpectedly, short cycling, or failure to maintain consistent heating throughout the home. This genuine Lennox OEM replacement matches original factory specifications for a reliable, drop-in fit in compatible Lennox heating systems.