How to Clean a Furnace Flame Sensor
A Dirty Flame Sensor Is a Common Service Repair
Why Does My Furnace Keep Shutting Off?
Does your furnace begin to start up, but then shut down just as it gets going? Does it do this a couple of times and then shut down for good? If so, you're likely having the same problem that many homeowners do every year—a dirty flame sensor.
Most service work on furnaces is complicated and needs to be done by an HVAC professional, but cleaning your flame sensor is a simple process. It's also easy to troubleshoot and see if it's the cause of your problem.
Though we will be using a standard gas-fired furnace for our example, boilers and other gas-burning appliances often also use flame sensors. The following steps could be used for these types of appliances too.
What Is a Flame Sensor and What Does It Do?
The flame sensor is a rather simple device located at the burner assembly. It’s not much more than a thin, usually bent, metallic rod that sits in front of the flame stream inside the furnace.
The purpose of the flame sensor is to confirm to the system that whenever the gas valve is open, a fire is actually present. If the unit kept on emitting gas when there was nothing to ignite it, a dangerous buildup of unburned gas would result. When your furnace begins to start up and the burners are ignited, the flame sensor has a very short window of time to detect the flame.
If the sensor doesn’t detect any flame, it automatically shuts down the unit. Most units will allow this shutdown three times before going into a "safety lockout" for about an hour before trying again. Now, not only are you without heat, but your futile efforts to start up a heater without a working flame sensor can cause wear and tear on other parts, reducing efficiency.
What Causes a Flame Sensor to Get Dirty?
It’s possible for a flame sensor to go bad; but more often than not, it is not broken, just dirty from carbon buildup. Because a flame sensor has a very low tolerance for variations in the reading it takes, the slightest coating of carbon can cause it to misread and shut down. Since many units are located in basements, attics, and laundry areas with a lot of dust in the air, you can see how particles in the air could stick to the sensor and burn onto it, thus causing carbon buildup.
What You Need to Make the Repair
- 1/4" hex driver or wrench (the tool could vary based on your type of mounting screw)
- Small piece of light grit sandpaper, steel wool, or emery cloth
- Dry, clean paper towel
1. Shut off the Power to the Furnace
Whenever you maintain your furnace (or other appliance), you MUST shut off the power to the unit. Turning off the thermostat (temperature control) does NOT shut down power to the furnace.
Usually, there will be a toggle switch (an on-off switch like a light switch) mounted on or near the unit. If not, you can shut the furnace down from the circuit-breaker box. Furnaces are supposed to be installed with dedicated breakers that can shut them down.
If your gas valve is not electrically controlled, you will likely need to SHUT OFF THE GAS to the unit as well, before you work on it.
There are many different types of furnaces and appliances. Should any of the information provided here not match what you see on your unit, STOP! Do not guess or assume anything when dealing with your heating system. Call an HVAC professional in your area to repair the unit.
Power Shut Off: Examples
2. Remove the Flame Sensor
The sensor is rather easily accessible and typically mounted by one 1/4" hex head screw. Upon removing this screw, the sensor will slide out to where you can more easily clean it. Carefully remove the sensor. Sometimes, not usually, you may have to detach the wire from the end of the sensor to give yourself more room to work.
A Wire in the Furnace Leads to the Flame Sensor
3. Clean the Sensor
Once you have removed the sensor, gently rub the metal rod (and nothing else) with a very light grit sandpaper. Remember, you're not sanding down an auto body here, just ridding the sensor of any buildup. Then, use a clean paper towel to wipe clean any dust left behind by the sanding.
4. Replace the Sensor
Once you've cleaned the sensor, simply reconnect its wire (if you've taken it off), remount the sensor on the burner assembly using the 1/4" screw, replace the door(s) on the unit, and turn the power back on.
5. Check Your Results
If the unit takes a few extra seconds to start up, or the fan immediately kicks on and runs for a bit, this is normal. Shutting down the power to the unit can cause it to reset and run through a short series of checks before trying to fire again. Once this check is complete, the unit should again begin to operate normally, turning itself off and on by command of the thermostat. Make sure the unit can start up and stay on at least one more time before being confident your problem is solved.
Replacing a Broken Flame Sensor
If cleaning your flame sensor did not work, it's possible that the sensor is broken, not dirty. You may need to replace it. To do that, you can follow these same instructions, except that no sanding is involved; just remove the old sensor and replace it with a new, functioning sensor. Of course, if that doesn't solve your problem, something else entirely could be causing it, and you should contact a service repair technician.
Buying a New Flame Sensor
It's not likely that your local hardware store will have the right part, but a contractor supply facility nearby may be able to help. There are also many web sites that offer flame sensor replacements. You will need the make and model of your furnace in order to find an acceptable match.
Video: How to Clean a Flame Sensor
Replacing Your Entire Furnace
If you're at your wit's end and you feel it's time to replace your furnace with a new model, it's important to know what to expect. If you are replacing a particularly old furnace you just might be able to save significant money on your heating costs every month—enough to eventually pay for the furnace.
I wrote the article Comparing the Cost of A New Furnace to How Much You'll Save to help those out who are looking to replace their furnace and want to know exactly what to expect. It has step by step instructions to help you figure out how much money you spend on heating, how much is wasted, and how much a new furnace can save on your gas bill.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
My furnace blinks once, lights and then quits. I cleaned the sensor, but keep getting the same results. Should I move on to the limit switch, pressure switch and circuit board?
Well, the sensor could be bad. It does go bad sometimes, and cleaning won’t help so perhaps it’s still the same issue. A pressure switch typically won’t let it light at all. The limit is a possibility. That would be my second suspect. Then I would check the board. But if everything else is firing in order, then I doubt that’s the problem.Helpful 54
What causes the furnace exhaust fan to shut down just before the burner ignites?
This could be caused by a pressure switch failure. This article might help you determine if that’s the case. https://dengarden.com/appliances/Everything-You-Ne...Helpful 22
I tried both cleaning my furnace's flame sensor and later installing a new flame sensor. However, the furnace still shuts off after only a few seconds of the burners operating. Do you have any ideas on what the source of the problem could be?
I'm really not sure based on this information as there are a lot of possibilities and some electrical testing would need to be done to narrow it down. Limit switch and pressure switch and circuit board would be my suspects.Helpful 18
Why would a new sensor get dirty right away and cause the furnace to quit?
Though it's not typical for this to happen, I would say a nearby laundry facility could produce lint which can float and burn, building dust on the sensor or perhaps "dirty" gas. Nothing else that I can think of.Helpful 14
My flame sensor mounting screw is rusty. What can I do get rid of that rust?
Outside of brushing it clean I don’t know. If it works it isn’t hurting anything. A 1/4”x1/2” long hex screw should be easy to find and replace it. The question I have is why is it rusty? You may want to keep an eye for water being where it shouldn’t.Helpful 7
© 2012 Dan Reed