A furnace flame sensor is a vital safety device that detects whether a gas burner has properly ignited by verifying the presence of a furnace flame sensor. One of the most common issues responsible for intermittent furnace operation is a dirty flame sensor, which prevents the furnace from detecting the burner flame appropriately; troubleshooting and cleaning a dirty flame sensor furnace can often restore normal heating function without expensive repairs. This guide covers everything needed to troubleshoot, clean, and replace troublesome furnace flame sensors.
A Flame Sensor and Its Role
A flame sensor rod sticks out near the gas burner flames inside a furnace combustion chamber. Its job is to verify that the gas burner did ignite when the furnace attempts to light.
This metal probe acts like an electrical switch that is activated by heat. When it senses a flame, it signals the furnace control board to keep the gas valve open and continue heating. If no flame is detected shortly after the burner ignites, it triggers the gas valve to close for safety to prevent raw gas from filling the furnace.
Signs Your Furnace Has a Dirty Flame Sensor
Short Cycling
If the furnace cannot detect a flame consistently, it will turn the burner on and then back off in rapid, repeated cycles. This usually sounds like the blower switching on and off multiple times per hour. Short cycling prevents providing adequate heating but also reduces furnace component lifespans.
Failure to Ignite
Excessive debris or corrosion on a flame rod can prevent it from sensing a small, newly ignited burner flame. This causes the furnace to shut off the gas supply to the burner prematurely as a safety precaution since raw gas should not continue flowing unburnt.
Without the flame verification that the sensor provides, the furnace control board does not keep the burner lit and fueled. So the furnace struggles to complete ignition, failing to start the heating process or cycling on and off repeatedly.
Ignition Without Heat
In some cases, the burner will successfully ignite and appear to operate normally, but combustion gases are not adequately heating the home due to the furnace shutting down prematurely.
Any furnace troubleshooting checklist should include checking for and resolving dirty flame sensor issues. For more information, read our article on how to test your furnace’s flame sensor.
Reasons a Flame Sensor Gets Dirty
Dust and Lint
Dust, pet hair, lint from laundry and bedding, and other similar debris floating through the air can slowly accumulate on the flame sensor rod over months or years of furnace operation. These particles get pulled into the blower and combustion air intake as part of the circulation airflow. As they pass by the sensor, some cling to and coat the metal probe.
Homes with indoor pets, carpeting, upholstered furniture, and cotton fabrics tend to suffer worse from this gradual debris buildup since more of these materials are present to release lint/dander into the air. If left unchecked, eventually, the flame sensor gets covered in a layer thick enough that its ability to react to the burner flame’s heat is impaired.
Oxidation
The metal alloys used to construct a flame sensor are subjected to intense, prolonged heat exposure inside a hot, enclosed furnace combustion chamber. Additionally, corrosive sulfur-containing compounds in some fossil fuels and water vapor byproducts interact with the metal sensor surface during gas burner combustion. Over years of cyclic operation, these conditions can cause oxidation, corrosion, and heat damage to slowly degrade and discolor the flame probe. The sensor loses its smooth, polished shine, transitioning to a rougher, darker surface.
As corrosion builds up, it insulates a portion of the metal from fully reacting to the flame’s thermal energy. This inhibits the sensor’s electrical switching function and ability to consistently confirm flame presence to the furnace control board during each heating cycle.
Oil Contamination
Many households neglect to replace their HVAC air filters regularly, or the existing filters are not sufficient for trapping airborne debris. Over time, accumulated dust gets sticky with oil, grease, and other vapors picked up from kitchen activities and cooking. Eventually, some of this oily film gets deposited via the circulating airflow onto surfaces inside the furnace, creating a tacky residue.
Without sufficient filtration, the flame sensor positioned adjacent to the moving air can get directly fouled by sticky buildup accumulating on its tip. Even a thin film or spots of greasy contamination on the sensor probe impair its ability to react to a burner flame attempting to ignite fully. Poor flame detection then causes short cycling or failure to start.
Professional cleaning or replacing these oil-fouled sensors restores ignition verification capability and proper furnace operation.
How To Keep Your Flame Sensor Clean
Homeowners can take proactive steps to avoid needing repeat flame sensor cleaning for their heating system.
Replace Air Filters Regularly
Clogged, dirty filters allow more dust and debris to accumulate throughout the entire HVAC system – including on the flame sensor probe positioned adjacent to moving air. Doing this will ensure your heating system is functioning properly.
Annual Furnace Cleanings & Safety Checks
An HVAC professional can fully sanitize blower assemblies, clean heat exchanger passages, inspect combustion chambers, and validate safety controls as part of yearly maintenance. This helps limit grime buildup and corrosion inside the furnace firebox.
Upgrade Filter Types
Consider installing a thicker pleated filter or adding supplemental filtration systems to the ductwork, like media cabinets or high-MERV standalone air cleaners. These reduce the dust and other airborne particles moving through the system and can improve indoor air quality.
In summary, identifying and troubleshooting common furnace problems like a dirty flame sensor can be done safely by most homeowners to minimize HVAC downtime and unnecessary repair costs. However, routine professional tune-ups and checks remain essential for optimal performance and equipment lifespan. Reach out to a technician for any complex furnace diagnosis or repairs.
The Logan Difference
As a family-owned company, Logan A/C and Heat Services provides exceptional customer service to all our customers throughout Ohio. Since 1969, we’ve been your go-to for all HVAC needs, including when you’re dealing with a dirty flame sensor on your furnace.
Our experienced technicians complete robust training on all major brands before servicing systems. We follow meticulous 20-point checklists overseen by knowledgeable install managers for each job. Staff communicate clearly and take time to demonstrate proper equipment set up and use after finishing. You get white glove treatment from our team. We stand behind quality work with continued maintenance support when required.
Contact us today to schedule your free estimate to see the difference Logan can make with your HVAC system, gas furnace, and more.
Frequently Asked Questions
Why does my gas furnace keep shutting off after a few minutes?
If a furnace shuts back down shortly after lighting, a dirty or failing flame sensor is often the issue. It cannot properly detect if the gas burner is lit. Cleaning the probe to remove dust, debris, and oxidation usually restores normal operation.
How do I know if my flame sensor is bad?
Signs of a defective flame sensor include the furnace repeatedly short cycling, not igniting at all, or shutting off before fully warming the home. Physically inspect the sensor for damage or excess debris buildup if these problems start occurring. Flame detection issues typically cause intermittent furnace operation.
Where is the flame sensor on a furnace?
Flame sensors are mounted near the burner assembly inside the heat exchanger combustion chamber. On older models, the probe sticks through an opening to be exposed to the burner flames. For newer sealed and high-efficiency units, the sensor is fully contained inside but positioned nearby where it can react to the ignition of gas jets.
Why does my furnace keep turning on and off?
Frequent short cycling of a furnace – switching back and forth from on to off repeatedly – is most commonly caused by a dirty flame sensor unable to detect if a flame is present after ignition consistently. Other possible causes include low gas pressure, faulty thermostat wiring, or sensor failure of a key component like the ignitor or control board.
How often should a gas furnace flame sensor be replaced?
Most flame sensors have a long lifespan and do not require regular replacement unless physically damaged or non-functional. Simple cleaning can often restore sensors to normal operation. Have an HVAC technician verify proper working conditions during periodic furnace inspections and tune-ups. Expect to replace a flame rod approximately every five years or so in most cases.
