If your gas furnace starts normally, the burners ignite and then the flame shuts off a few seconds later, the flame sensor may be dirty or failing.
The flame sensor is a small safety component that confirms the burners have actually ignited. If it does not detect a flame, the furnace shuts off the gas supply to prevent unburned gas from continuing to flow. A bad or dirty flame sensor can make it look like your furnace is trying to run, but cannot stay on.
What does a furnace flame sensor do?
The flame sensor is usually a small metal rod positioned near the burners.
After the burners ignite, the sensor detects the flame and sends a signal to the furnace control board confirming that combustion is taking place. If the control board does not receive that signal, it assumes the burners failed to ignite and shuts the gas valve. That is an important safety function.
The burners light, then shut off quickly
This is one of the most common signs of a flame sensor problem.
You may hear the furnace begin its normal startup sequence:
The inducer motor starts
The igniter activates
The burners light
The burners shut off a few seconds later
The furnace may then wait and try the startup sequence again. If the burners consistently ignite and then go out almost immediately, the flame sensor is one of the components that should be checked.
The furnace keeps trying to restart
A furnace with a flame-sensing problem may repeatedly attempt to start.
You may hear it cycle through the ignition process several times before eventually shutting down or going into a lockout mode. This can sometimes make homeowners think the thermostat is malfunctioning, but the real problem may be that the furnace is not receiving a reliable flame signal.
Your furnace may stop heating altogether
Many furnaces are designed to stop attempting ignition after several unsuccessful tries.
When this happens, the furnace may enter a safety lockout and stop operating for a period of time. Some systems will try again later. Others may require service or a reset after the underlying problem has been corrected. If your furnace repeatedly enters lockout, there is a reason. Resetting it over and over without diagnosing the problem is not a good solution.
A dirty flame sensor is more common than a failed sensor
Flame sensors operate directly in the burner area, so they are exposed to heat and combustion byproducts every time the furnace runs.
Over time, oxidation or residue can build up on the sensor rod. That buildup can interfere with the very small electrical signal the furnace uses to verify the flame. As a result, the sensor itself may still be functional but unable to reliably detect the burner flame. Cleaning the sensor can sometimes restore normal operation, but similar symptoms can also be caused by other furnace problems.
Can you tell a flame sensor is bad just by looking at it?
Not necessarily.
A flame sensor may appear dirty, discolored or oxidized and still be functional after proper cleaning. A sensor can also look relatively normal but fail to provide the correct signal. The most reliable diagnosis involves observing the furnace’s operating sequence and testing the flame-sensing circuit with the proper equipment. That helps distinguish a bad sensor from another problem that happens to produce similar symptoms.
Other problems can look like a bad flame sensor
A furnace that lights and then shuts off does not automatically have a bad flame sensor.
Similar symptoms can be caused by:
Poor burner grounding
Damaged or loose wiring
A control board problem
Burner contamination
Improper combustion
Gas supply problems
Ignition system problems
That is why replacing the flame sensor based on symptoms alone is not always the best approach. A technician can test the sensor and determine whether the problem is actually the sensor, contamination on the sensor or another part of the furnace.
Can you clean a furnace flame sensor yourself?
Some homeowners do clean flame sensors themselves, but the sensor is part of the furnace’s combustion and safety system.
The furnace should be shut off properly before any internal components are touched, and the sensor can be damaged if it is handled or cleaned incorrectly. There is also a larger issue: cleaning a sensor may temporarily make the furnace run again without proving why the flame signal was weak in the first place. If you are not comfortable working inside a gas furnace, having the system professionally diagnosed is the safer choice.
How long does a furnace flame sensor last?
There is no fixed replacement schedule for a flame sensor.
Some last for many years, while others may develop problems sooner depending on furnace operation, combustion conditions and contamination. Flame sensors are commonly inspected and cleaned during furnace service because buildup can develop gradually over time. If a sensor repeatedly causes problems even after proper cleaning and testing, replacement may be appropriate.
When should you call for furnace repair?
Call for service if your furnace:
Lights and shuts off after only a few seconds
Repeatedly attempts to ignite
Frequently goes into lockout
Will not stay running
Has already had the flame sensor cleaned but the problem returned
Shows other unusual symptoms such as strange odors or abnormal burner operation
A qualified technician can test the flame sensor signal and check the rest of the ignition and combustion system at the same time.
Need help with a furnace flame sensor problem?
If your furnace lights but will not stay running, the flame sensor may be dirty or failing, but it is not the only possible cause.
Roberts Brothers Comfort technicians can inspect the ignition system, test the flame sensor and determine what is preventing your furnace from operating normally. We provide expert furnace repair service like this throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
