Locating the Lennox furnace flame sensor is the first critical step in diagnosing intermittent shutdowns or failure to ignite. This small, typically metal or ceramic component acts as the guardian of the burner, ensuring the gas valve only remains open when a consistent flame is present. While the exact positioning can vary slightly between models, the sensor is always positioned very close to the burner assembly it is meant to monitor.
Understanding the Role of the Flame Sensor
The flame sensor is a safety device that verifies the presence of a flame once the gas valve opens. If the sensor fails to detect a flame within a few seconds, the furnace will shut down as a safety precaution. Because it is a passive component that relies on conductivity, the sensor can become coated with dust or corrosion over time, leading to false readings. Accessing it requires knowing precisely where to look.
Primary Location Within the Burner Assembly
For the majority of Lennox gas furnaces, the flame sensor is located directly adjacent to the gas burner. You will typically find it mounted to the side of the burner head or connected to a small bracket near the igniter. It is positioned so that the tip of the sensor—usually a thin metal rod or a porcelain-coated cylinder—faces the path of the flames coming from the burner ports.
Visual Identification Tips
Look for a small metal rod, often with a ceramic tip, protruding into the burner chamber.
It is usually connected to a wire harness with two terminals.
The sensor is positioned closer to the gas valve than the igniter, but still within the combustion zone.
Accessing the Sensor for Maintenance
To clean or test the sensor, you must first turn off the power to the furnace at the thermostat and the breaker box. Removing the access door to the burner compartment will reveal the sensor’s location. Because the sensor is often threaded into place, you may need to loosen it carefully with a wrench to inspect the tip for soot or grime buildup that can impede its function.
Variations Across Lennox Models
While the core location remains consistent, the specific housing can differ. In older two-stage models, the sensor might be mounted vertically near the igniter. In newer high-efficiency units, the sensor may be integrated into a smaller, more compact burner module. Consulting the specific model diagram on the access door or the unit’s data plate can clarify the exact orientation if visual inspection is difficult.
Troubleshooting Connection Points
Beyond the physical location, the connection between the sensor and the control board is just as important. Corrosion at the wire terminals can mimic a faulty sensor. When troubleshooting, inspect the wire harness for cracks or loose connections before assuming the sensor itself needs replacement. Ensuring a solid electrical path is essential for accurate flame detection.
When to Call a Professional
If you are uncomfortable working with gas lines or high-voltage components, locating the sensor is only the first step. A qualified HVAC technician can use a micro-amp meter to test the sensor’s output and determine if cleaning will suffice or if a replacement is necessary. Proper diagnosis prevents unnecessary part replacements and ensures the safety of your heating system.
