Lennox 73K18 Limit Switch

The Lennox 73K18 Limit Switch is a critical genuine OEM replacement part designed for precise thermal protection in Lennox, Armstrong, and Ducane heating systems. This 140-170°F automatic limit control functions as a normally closed (NC) disc-style switch, serving as a primary safety "guardian" for the heat exchanger. When the internal plenum temperature reaches the 170°F cut-out point, the circuit opens to shut down the burners, preventing equipment damage and ensuring safe operational temperature rise. Once the system cools to the 140°F cut-in point, the automatic reset mechanism restores the circuit for continued duty.

This TOD Series 60T limit control is a direct cross-reference for part number 73K1801 and is frequently found in G23 and G24 series furnaces. For field technicians diagnosing short cycling or limit switch error codes (such as E250), this OEM component ensures the exact calibration required to match factory airflow specifications. Replacing a failed primary switch with a Lennox 73K18 maintains the integrity of the safety string, protecting against overheating caused by restricted airflow, clogged filters, or blower motor failure.

• Direct Compatibility: Exact Lennox 73K1801 replacement for a seamless "drop-in" fit during emergency service calls.
• Precise Thermal Calibration: Fixed 140-170°F temperature range prevents nuisance tripping while providing reliable furnace over-temperature protection.
• Automatic Reset Functionality: Eliminates the need for manual intervention once the temperature returns to a safe 140°F cut-in level.
• Industrial Build Quality: High-grade disc-style Lennox HVAC part engineered for high-cycle durability in residential and commercial applications.
• Safety Compliance: Genuine OEM part ensures the system operates within its original UL-certified safety parameters.

What causes a Lennox 73K18 limit switch to trip repeatedly?
Most trips are caused by airflow restrictions. Check for dirty air filters, blocked return air grilles, or a malfunctioning blower motor. If the temperature rise exceeds the furnace nameplate rating, the 73K18 limit switch will open to protect the heat exchanger.

Is the 73K18 interchangeable with the 73K1801?
Yes, the 73K18 and 73K1801 are functionally identical and shared across the Lennox family of brands, including Allied Air, Armstrong, and Ducane.

How do I test if this limit switch is defective?
With power disconnected, use a multimeter to check for continuity across the terminals. Since this is a normally closed switch, it should show nearly zero resistance at room temperature. An open circuit at room temperature indicates a failed switch that requires replacement.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.