How to Clean A Furnace Flame Sensor

A Dirty Flame Sensor Is a Common Service Repair

A typical flame sensor. Some have their tips bent at a different angle, or not bent at all.
A typical flame sensor. Some have their tips bent at a different angle, or not bent at all. | Source

Why Does My Furnace Keep Shutting Off?

Does you furnace begin to start up, but then shut down just as it gets going? Does it do this a couple 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.

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.

Author's Qualifications

Cre8tor has 15 years of experience in HVAC Installation and Service, an EPA certification, and coursework in electrical theory, thermodynamics, and refrigeration.

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

Let's get started...

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

A toggle switch that has been mounted in the ceiling near the furnace.  Your toggle switch may be on the furnace, or you may have to shut off the unit at the circuit-breaker panel.
A toggle switch that has been mounted in the ceiling near the furnace. Your toggle switch may be on the furnace, or you may have to shut off the unit at the circuit-breaker panel.
A typical breaker panel, where you may also be able to turn off the power to your furnace.
A typical breaker panel, where you may also be able to turn off the power to your furnace.

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

This furnace has four burner ports. Between the first and second burner ports (from left to right) you can see the wire leading to the flame sensor.
This furnace has four burner ports. Between the first and second burner ports (from left to right) you can see the wire leading 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

© 2012 Daniel Robbins

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Comments 34 comments

Kristen 8 days ago

Thank you so much for this post!! It's awesome! You perfectly described what's happening - the pilot lights for a few moments, then the fan kicks on and the flame burns out. I am no handy lady by trade but I do like to try things myself to save money. I wrote some contactors describing my issue but none of them called me back for an appointment and I'm so glad!

I removed the sensor and it didn't look very dirty, but I rubbed it lightly with sandpaper and wiped it down. Unfortunately, cleaning didn't solve my problem, but your post gave me the confidence and direction needed to order a replacement sensor and replace it myself. My dad and boyfriend will be so proud!!!

rking96 profile image

rking96 8 months ago from Charleston, SC

Just had this problem fixed in my furnace yesterday. Wish I had read this 2 weeks ago. Would have saved me $150 literally. This looks very thorough and easy to follow.

Cre8tor profile image

Cre8tor 10 months ago from Ohio Author

Jody, Sorry for delay but you definitely need a service tech out there. If you've had problems since you got it, it sounds like the regulator isn't set properly and you again, DEFINITELY need a tech to look at your situation before it gets worse. There are sensors that should be monitoring where the flame is as well and it doesn't even sound like those are working. There is definitely something not right there and sounds dangerous.

Jody 10 months ago

Can someone help me. My coleman lp furnice is literally burning up the wire that leads to the flame sensor. I feel it is way to hot in there. It cooks off outside then burns wire black so they literally break when moved. Obviously this is a problem. I cut off bad part slid it down reattached it. It took 36hrs to burn through that work. I own trailer but rent lot space. Our propane is on an old meter that is fed through the park to us from one of two lp tanks. I have had same furnace same stove both like 5 years old. With all the flame that is not blue in there what do I do? He controls everything and we live in rural area in the county but who can one report too? The other 2 wires to flame igniter are white and have that aspestos covering. I have not seen anywhere on pics in the net that have all 3 in same hole. It's like it's to close. Please help it's a dang çoleman lp mobile home furnace that I don't want blowing up. I have had problems with it since I got it. I didn't buy it county weatherization people did. Please help

Cre8tor profile image

Cre8tor 19 months ago from Ohio Author

If it runs for 30 minutes, no. It's not a flame sensor and that is too long for a unit to run in a single cycle for sure. Sounds more like a high limit switch. The unit runs too long and trips the temperature limit so the fan runs to dissipate heat. The stat won't respond because the circuit board has overridden it as a safety default. That said, you have a different problem and I would start with the stat in my investigation. Sounds like it's may not be functioning properly.

linus 19 months ago

Thanks for the info. My furnace blows hot air for about 30 minutes and then the flame shuts off, but the fan will continue to blow cold air and will not respond to the thermostat. I have to shut it off at the breaker or unplug it. While it is blowing hot, it will respond to the thermostay, but after the flame shuts off, the thermostat doesn't control anything. Could this be a flame sensor problem?

Cre8tor profile image

Cre8tor 20 months ago from Ohio Author

yik - Unfortunately, no. I'm not sure what to offer other than trying to clean the sensor or change the thermocouple if it's a standing pilot. These are easy and cheap. If it fixes it, great and if not, you're no worse for the wear really and then pursuing a more expensive repair will be with less guesswork. It is very hard to diagnose intermittent issues when the unit is working while you're inspecting it.

yik 20 months ago

My boiler (Goodman) works fine for a few hours, cycling on and off, and then it doesn't start anymore. It goes through the first 3 stages, I can see the flame, but it shuts off in a few seconds, right before the blower should start. It starts working again in 2-3 hours. And it goes like this for a couple of weeks. When HVAC guy came - it worked, so he didn't know what to do. He says it is probably the control board... It seems to me like the flame sensor is bad, but why would it work for some time - and then doesn't? Any idea?

Cre8tor profile image

Cre8tor 21 months ago from Ohio Author

Kim - So nice to read comments like this. Thanks for the feedback and allowing me to feel useful today.

Kim 21 months ago

thank you so much! I am not very handy, and not very rich :) I did this easily and you saved me almost $300

Candice Harding 2 years ago

Great hub! So many heating and air conditioning problems seem to come out of lack of upkeep than anything else. I learned the hard way to make sure I stayed on top of air conditioner and furnace cleaning. I've had each of them shut off and later discovered that it was a simple maintenance issue.

Candice Harding

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

@MohZ - You are welcome! I'm so glad to get feedback like this since it is my goal to help people in situations like yours. Glad I could be of assistance and thank you so much for the feedback!

MohZ 2 years ago

Thank you, thank you thank you! Your description of the symptoms was spot on. Saved me who know how much $$$, and more importantly, it's -1 with a low of -18 tonight, and I doubt I could get a guy out here right now.

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

Tf81- if it needs cleaned that often, something is up. What that is??? Replacing it shouldn't be to costly and is what I would do first at this point but the continuous build up is caused by something burning up on it. Perhaps lint if a dryer is near by or something like that. I've seen where underground leaks can allow moisture into the line and cause problems like this too. Let's hope not. Could just be that the integrity of the part is shot though in which case you're on the right track. Let me know how it turns out. Thanks for reading.

tf81 2 years ago

I just missed the edit window. Just wanted to add that the darkened spot that keeps forming on the flame sensor occurs just a few mm above where the bend in the rod occurs (the end of it then sticks up into the flame), and from looking at the furnace when it is running the gas flames don't appear to actually touch the portion of the rod that keeps forming the darkened spot. So this seems very strange to me.

tf81 2 years ago

My flame sensor needs to be cleaned every 24-36 hours or less in order for the furnace to work properly, as a black stripe of material keeps forming on the exact same spot over and over again. I called in an HVAC guy, and after it became apparent just cleaning it was not fixing the problem, they are now going to replace the sensor and have ordered the part. Will this even solve the problem though? Could it be something is built up on the burner and causing it to darken in the exact same spot? I mentioned this to the repair guy, but he pretty much just ignored me like I have no clue what I am talking about, and insists replacing it should fix the issue. When a flame sensor is malfunctioning, does it blackening in one area point to it needing to be replaced? Thanks.

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

Dawn - There's likely not too much you can do but clean it. If you're cleaning it and it's working...quit paying people to come and replace it. It's more likely the dusty, perhaps "linty" room if there's a laundry there and often, it's the gas itself that's leaving the residue. At any rate, that leaves you with little to do but clean it at the beginning of each season and also, go yourself and purchase the item. It's so simple to replace and you can likely find them at your local hvac supply house rather cheaply. Often, this can be found in a universal type part if you can't find the exact manufacturer's replacement.

Dawn 2 years ago

I have to clean mine ever three to four months. I was directed to use steel wool by the company.

How do I fix this problem. Hot water is hit or miss in my house and the company after replacing the parts repetitively until the warenty ran out is useless

My water heater is in the basement mechanical room. How can I make it so nothing can gum up that sensor?

Cre8tor profile image

Cre8tor 2 years ago from Ohio Author

Your welcome. That's my intent. Save the world one heat call at a time.

Tim 2 years ago

Thank you very much, just saved me a service call and a cold family.

SamitaJassi profile image

SamitaJassi 3 years ago from Chandigarh

Thanks for the above tips. Voted up and useful.

Michael Potter 3 years ago

Thanks for the tips! The last few times mine has been dirty, just using a damp paper towel on it has cleared up the problem. It doesn't even look like I'm getting anything off of the sensor, but sure enough, when I put it back in, it start operating normally.

Cre8tor profile image

Cre8tor 3 years ago from Ohio Author

I to have used a dollar and it works fine...for those who still know what money looks like. :-)

peter 3 years ago

i work in the plumbing and hvac industry, when it comes to flame rods i always use a one doller bill, sand cloth can damage the flame rod but different strokes for different folks

Cre8tor profile image

Cre8tor 3 years ago from Ohio Author

Thank u for the feed back. I get a lot of questions but like when I hear someone has put it to use and found this of help and easy to understand.

mike 3 years ago

this was great info. I don't consider myself mechanically inclined but I was able to do this. If I can do it - anyone can.

allanmullaly profile image

allanmullaly 3 years ago


liamm1320 3 years ago

Thanks for sharing this. It really helped. I keep thinking we should probably get a new furnace, I'm just not sure what the best place to get one is. We were thinking about getting one here: what would you suggest?

Cre8tor profile image

Cre8tor 3 years ago from Ohio Author

BT - I'm glad you say this because I've heard this however what's funny is that I've cleaned mine every year with a light abrasive for the last 9 years as part of maintenance and never had a problem. I've performed this service for others on yearly service contracts and problem. So from my first hand experience, I see no problem. I also clarify were not sanding out a car body. In the trade, I've seen steel wool, dollar bills, toothbrushes, and more used for this task....I suppose I like my method. So for those reading, I'll leave it up to you to determine what you prefer but I've not misled you here.

bt 3 years ago

you're wrong, by using any abrasive on the rod you will remove the glossy finish leaving grooves in the rod only to attract more debris in the future. wipe it clean or replace it.

nick 3 years ago

Thanks - described my symptoms perfectly and fixed it easily!

Cre8tor profile image

Cre8tor 4 years ago from Ohio Author

HVAC can offer a lot of possibilities. Watch it fire up next time. Make sure there's no abnormal flaming (roll out). Check your filter and for weak ductwork that's "oil-canning". I'd hate to diagnose beyond that. Thanks for reading and glad I could help.

jenubouka 4 years ago

Great hub! I had constant problems with my furnace's flame dying out to the point I knew the gas man's first name and as your awesome hubs states it was dirty.

I have a question: Lately my furnace makes a thud sound whenever it is about to turn on and right after it turns off, like a baseball bat hitting the side what is that?

larryriegle profile image

larryriegle 4 years ago

nice job on this information...very thorough...i am a home remodeler by trade and agree that this is good advice and information.

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    Daniel Robbins (Cre8tor)318 Followers
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    Dan has been in the HVAC industry for 22 years with experience in aspects ranging from installation and service to sales and distribution.

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