Locating the Lennox furnace flame sensor is a fundamental step for any homeowner or technician aiming to troubleshoot intermittent ignition failures or consistent lockouts. This small, often overlooked component plays a critical role in the safe operation of your heating system by confirming the presence of a flame whenever the gas valve is open. If the sensor fails to detect the flame, even momentarily, the furnace will shut down as a safety precaution, leaving your home without heat and prompting a search for the source of the problem.
Understanding the Role of the Flame Sensor
The flame sensor acts as a safety verification device within the burner assembly. When the thermostat calls for heat, the inducer motor creates draft, the ignition source lights the gas, and the sensor detects the heat or light generated by the flame. Its primary function is to ensure that gas is not continuously released without combustion, which would create an unsafe and explosive environment. Over time, the sensor can become coated with soot or corrosion, preventing it from accurately detecting the flame and causing the system to cycle on and off repeatedly.
General Location Within the Furnace Cabinet
To begin the search, you must first access the main furnace cabinet, which is usually located in a basement, utility room, or mechanical closet. Once the power is safely turned off at the breaker, remove the access panel to expose the interior components. Inside, you will see the burners, the gas valve, the igniter, and the draft inducer. The flame sensor is positioned very close to the burners, specifically oriented to "see" the flame as it emerges from the burner ports.
Position Relative to the Burners and Gas Valve
Specifically, the sensor is typically mounted on the side of the burner assembly, facing the path of the flame. It is often secured with a small bolt or screw and features a thin, metal rod protrusion made of stainless steel. This rod is the sensing element and must be positioned directly in the path of the flame for optimal function. You will usually find it situated near the gas valve itself, as the valve controls the flow of fuel that the sensor is monitoring.
Visual Identification and Physical Characteristics
Identifying the component correctly is essential before attempting any cleaning or replacement. The Lennox flame sensor is generally a small, cylindrical device with a ceramic or metal body. It is connected to the furnace control board via a two-wire electrical connector. The tip of the sensor is the active part; it is usually a straight or slightly bent metal rod that extends into the burner chamber. Unlike the spark igniter, which creates a spark, the sensor does not move and remains stationary throughout the operational cycle.
Common Appearance Across Models
While specific placements can vary slightly depending on the model year and series, the physical layout remains consistent across most Lennox units. The sensor is almost always located on the upper portion of the burner frame. Look for a metallic rod positioned approximately one to two inches from the burner orifice. It is often situated opposite the igniter, allowing it to capture the full flame front as it rolls across the burner surface during ignition.
Troubleshooting Based on Location
Once you have located the sensor, you can determine if it is the source of the problem. The most common issue is a thin layer of soot or oxidation that builds up on the sensing rod, insulating it and blocking the flame signal. If the sensor is dirty, the furnace may attempt to ignite, fail to sense the flame, and then shut off immediately. Cleaning the sensor with a soft emery cloth or steel wool can often restore functionality without the need for costly part replacements.
