Locating the Lennox flame sensor is the first critical step in diagnosing a no-heat call for heat, especially on older furnace models. This small but vital component acts as the guardian of the burner flame, ensuring the gas valve only remains open when a stable flame is present. Without a clear signal from this sensor, the furnace will fail to ignite or will shut down prematurely as a safety precaution. Understanding its precise position on your specific Lennox furnace model is essential for both homeowners performing basic checks and HVAC technicians executing a thorough service sequence.
Understanding the Role of the Flame Sensor
The flame sensor is a safety device designed to verify the presence of a burner flame in gas-fired heating systems. During a standard heating cycle, the inducer motor creates a draft, the gas valve opens, and the ignition source—whether a hot surface ignitor or a pilot light—creates a flame. This flame generates a small electrical current that travels through the flame sensor to the control board, confirming ignition success. If the control board does not detect this current within a few seconds, it assumes a failed ignition and initiates a safety shutdown, often locking out the system until a reset cycle occurs. This safety protocol prevents unburned gas from accumulating inside the heat exchanger.
Standard Location on Lennox Gas Furnaces
On the majority of Lennox gas furnaces, the flame sensor is positioned directly adjacent to the burner assembly. You will typically find it mounted to the side of the burner, featuring a thin metal rod approximately one to two inches in length, protruding into the path of the flames. This rod is often connected to a wire harness that routes back to the control board, usually located at the top or side of the blower compartment. To access it, you will generally need to remove the front service panel and potentially the burner assembly door, depending on the specific configuration of the unit.
Visual Identification Tips
Look for a small metal rod located very close to the gas valve and the burner ports.
The sensor is usually attached to a thin, two-wire cable that connects to a terminal on the control board.
It is often secured with a metal bracket or clip to the burner cabinet to prevent movement during operation.
The tip of the sensor is usually covered in a ceramic or metal housing to withstand high temperatures.
Common Placement by Furnace Series
While the general location remains consistent, the exact orientation can vary slightly between Lennox product lines. In many older models, such as the Merit or older Versa series, the sensor is often mounted horizontally on the side of the burner, requiring removal of the burner assembly for full access. Newer high-efficiency models, like the Prestige or some Infinity series, may integrate the sensor design differently due to compact heat exchanger configurations. Always refer to the specific service manual for your unit’s model number to confirm the layout before attempting disassembly.
Troubleshooting and Maintenance
Over time, the flame sensor can accumulate dust, soot, or carbon buildup, which insulates the metal rod and prevents the proper conduction of the micro-amp signal. This results in intermittent shutdowns or a complete failure to ignite, even when the gas supply and ignition are functioning correctly. Locating the sensor is the first step in cleaning it; once found, the rod can be gently cleaned using fine-grit sandpaper or emery cloth to restore its conductive surface. It is crucial to handle the component carefully, as the rod is relatively fragile and can break if subjected to excessive force.
