My gas furnace is only a couple years old, so I wasn’t real happy when our gas furnace began operating intermittently, sometimes starting when it should and sometimes not. We would be sitting in the living room playing a board game or just walking around and one of us would say, “Hey, it’s kind of chilly in here…” Essentially, the heat would be working fine for several weeks and then one day it just wouldn’t start. We’d sometimes hear the furnace try to kick on, but it wouldn’t last very long before it shut down again. Our gas furnace was basically cycling through its “starting cycle” much too quickly and it was never actually running for more than a few minutes. If this sounds like your gas furnace problem then read on to see what you can do to fix it before making an expensive service call to a heating repair company.
Our “easy” fix for this problem would be turn the heat off at the thermostat, wait about 10 to 15 minutes and then turn the heat on again, hoping for the best. Sometimes that would work and the heat would kick on, but sometimes it wouldn’t. I went through all the basic gas furnace troubleshooting steps and I was still averaging about a 50% rate of being able to get the heat going again. Sometimes, but not always, our thermostat would give us a little warning light telling us that something was wrong with the furnace.
Now, you generally don’t want to go tinkering around with your own gas furnace unless you’re pretty confident in your ability to fix things. It’s actually pretty difficult to get the full operating manual for most gas furnaces because companies don’t want you taking apart an appliance that uses mixes natural gas with ignition sparks. That makes sense. You don’t want to hurt yourself or blow up your home accidentally. That being said, there are some basic things you can do on your gas furnace to keep it running smoothly. You can clean it regular (a vacuum will do), you can change you home’s furnace filter and you can do simple thing like make sure nothing is blocking the air flow around your furnace and make sure your electronic thermostat has fresh batteries.
That’s when I remembered it: the flame sensor! The flame sensor is a simply safety device that can be found in all modern gas furnaces. Though it sometimes prevents your gas furnace from turning on when you want it, it also protects people and homes from dangerous or deadly gas leaks.
How A Gas Furnace Works
The flame sensor in a gas furnace should be cleaned from time to time.
The flame sensor then checks to see if there’s a gas flame, measures how hot the flame is and helps the furnace regulate how much gas to use to keep things warm. When it’s working properly the gas flame fires right on the flame sensor, telling your furnace and the flow of natural gas has been properly lit and that the air in the furnace is getting warmed. So as long as the flame sensor says “YES!” the furnace knows the gas is burning off and everything is working.
The problem comes in when the gas flows, the igniter begins to spark and the flame sensor does NOT detect a valid flame, even if there is one. If the furnace thinks that natural gas is flowing but it’s not igniting properly, then we have a problem. Natural gas could fill the house and explode with a spark from somewhere else or it could simply poison all the residents in the home. Neither situation is good.
The Flame Sensor
So furnace manufacturers use this flame sensor as a safety measure. If the sensor think the flame is not present (indicating flowing gas with no flame to burn it away) then the furnace will go into “lockout” mode because that could be the sign of a gas leak or other potential problem. The last thing want is a furnace trying to spark in a home with gas leak. This lockout mode could last anywhere between 15 to 30 minutes. You can sometimes bypass the wait by flicking the circuit breaker or resetting the electrical power to the furnace. No, you’re really not supposed to do that, so if you are unsure about anything with your furnace you’re better off calling in a technician.
Over the years a flame sensor will get a bit dirty or even corroded and it won’t detect that a flame is present, even when it is. I’ve watched my gas furnace spark, watching the blue flame burn fine for about two minutes and then seen my furnace instantly shut itself down, all because the flame sensor didn’t properly feel that hot flame on it!
Fixing A Gas Furnace That Starts Intermittently by Cleaning the Flame Sensor
Fortunately, the fix for a gas furnace that starts intermittently or doesn’t start at all because of a dirty flame sensor is pretty easy: clean the sensor with some steel wool something metal to gently scrape it. There are a few steps to follow:
1. Turn off your heating system: You should turn off your heating system both at the thermostat and then again at the breaker or shut-off switch. You’re going to be taking the cover off your furnace and poking around in there and you don’t want to risk getting electrocuted.
2. Locate your furnace’s flame sensor: Some furnaces have more than one. The sensor is located somewhere around your gas burners and it’s usually pretty easy to spot. Most are metal or gray rods that are about 3 to 4 inches long with a while ceramic base. Some will look like a little metal spatula. The sensor can be either straight or angled but it should be positioned right in front of one of the burners.
3. Remove the flame sensor (optional): In some furnaces I’ve noticed it’s nearly impossible to easily get to the sensor, which means you’ll probably want to remove it. There are often one or two little screws holding it in place. You don’t need to unplug the flame sensor (there’s probably a wire coming out the end) you just need remove the screws so you can get to the grey metal rod part. If you remove the flame sensor be sure to take note of where it is because you’re going to want to position back in the same spot that you removed it from.
4. Clean the sensor: Again, you can either use steel wool to clean it or you can try to simply scrape it down a little bit with a screwdriver or other metal tool. Some people suggest using emery cloth or sandpaper, but I don’t recommend that because they can both leave chemicals and other residue on your flame sensor. You basically just need to remove some of the crud and corrosion that’s built up on it over time so that it properly detects the flame from your furnace.
5. Reassemble everything and test: That’s it! Put everything back together, turn your heating system back on (at the breaker and the thermostat) and then sit back and wait a few minutes for it to start up.
Furnace Repair / Flame Sensor Cleaning Video
Flame sensors can go bad and they sometimes need to be replaced (you can even buy gas furnace flame sensors on Amazon
And that was it! Once I got everything back up and running the problem was solved – my gas furnace has now been running without a hitch for the past seven days, all because it has a clean flame sensor. By the way, I learned this trick from calling an HVAC repair guy a few years ago. When I told him my problem he opened up my furnace, scrubbed the flame sensor with some steel wool for about 30 seconds and then put the cover back on my system. Two minutes later the heat kicked on. He charged me $150 for five minutes of work, so by cleaning your own furnace flame sensor you can save yourself a fair bit of money!
You just saved me a call to the hvac repair man. Just got some steel wool, cleaned off that little metal rod, fired up the first time. Thanks
This worked for me several times over the years. My furnace LED code error message was something like “Call for heat, but no flame detected.” So if there is a call for heat (i.e., gas) but the sensor does not detect sufficient flame, it will send a signal to shut off the gas flow so my house doesn’t fill with un-ignited gas. Each time it happens you should hear the furnace blower motor for about ~30 seconds before, the actual furnace flame jets for 5-10 seconds, the flame jets stop, and the blower runs for another ~30 seconds.
I haven’t used light steel wool, but I’ve had luck with a light sandpapering, and even used a pencil eraser. The fix seems to work for months and usually gets me through the rest of the heating season.
I’ve done this for years, which is probably both good and bad. It’s an easy fix after I did it once, and it was even easy in a second house because I knew what to look for. If I knew a new flame sensor would fix it forever, I’d do that. But in the meantime, I have plenty of pencil erasers.
This actually works, would not have believed it but here is what happened to me.
I woke the Sunday morning after Christmas to find the house unusually chilly. The thermostat read 60 degrees and indicated the heat was on, but it was not! We had family coming the next morning and the furnace not running on a weekend, is how my luck typically goes. I cycled the power to the furnace, it ran flames for a few seconds and then off. Next I did what has become the norm these days for any issues, whether it be with the house or my car or anything, I searched the internet and ran across this web site and this article. When I removed the flame sensor, it had a light white powder or surface corrosion on it. I followed what the article said about cleaning the flame sensor with steel wool and wallah!! After re-installation, the furnace fired up and heated the house on the first try!!
Thank you thank you thank you.Easy to do and the furnace is working great.
This just happened to my furnace. Flames on for 15 or so seconds, then off, and it would not heat the house. I took out the little flame sensor and cleaned it off with rubbing alcohol and a pencil eraser, put it back in, problem solved. Gas repair guys (Barron Heating) wanted to charge me a hundred bucks just to come out and look!!!
On another note, I also fixed another problem with info from here a while back- the little heater element that ignites the gas was cracked and from info gained here I was also able to repair that (although the part cost was around $70 at the time, ridiculous!)
Anyway, thanks to all of you for helping me save money I can’t afford to spend!
This is exactly what my furnace is currently doing. It is WAY too cold for this. 14 degrees at the moment. I have already tried removing and cleaning the flame sensor, but it is still not running right. Thank god I have a wood burner in my basement! I will try cleaning the flame sensor again after reading from this website and hopefully I will get better results. Either way, thanks for the info! It is good to know that there is good help out there.
I was having similar problems–the furnace would work fine for awhile, then the house would get to 59 or 60 degrees even though the thermostat was set at 65. A couple of times i went down and checked the furnace and flipped the on off switch on and off and it’d fire right up. a week later I was having similar problems and did it again and again it worked for 1 heating cycle, then it wouldn’t come back on. I found the flashing code that indicated heat sensor issues and came to this web site. I pulled out my sensor and cleaned it with fine sandpaper and reinstalled it and viola–the flashing led lights went back to normal and the furnace is working fine so far! Thanks a million (and $145)
My furnace was experiencing same symptoms. Cleaned flame sesnor with steel wool. It didn’t really look dirty–but it worked!! Furnace fired right up. Thanks–now we won’t freeze tonight!
Awsome Blog!
Same thing here don’t know jack about furnace but looked online and u tube and found out that the sensor needed replaced or clean so I cleaned it with a pair of pliers and now is working fine still gonna order a new part I’m not always home so can’t expect the wifey to do it and then I came across this interesting article wish I would gave read this first instead of going crazy with out heat lol happy holidays
Thank you so much for this article and pictures. Woke up to cold house and restarted my furnance by flicking circuit breaker . It would run for 30 secs and shut off. Although manual says Falme sensor , I had no idea it was same thing as thermocouple. Thanks for the pics. Now I now what it is. Cleaned it and now running, hopefully for good.
I have a Mr. Heater HSU75LP garage heater that is driving me nuts. I live in Minnesota where it gets real cold. When I want to work in my garage I turn on Mr. Heater. (I turn it on by using the wall switch). The STAT (programable)is set at 60′. The blower comes on and after about 30 sec. I hear a click (igniter)and the flames come on, sometimes. If I try turning the wall switch on and off it may light off after 2, 3, 4 or 5 times or not at all. I will go out later and go thru the same process and it lights off. After it lights it seems to operate fine for as long as I leave the main switch on. The red blinking light pulses fast and steady at all times regardless if furnace lights or not. I thought of checking the flame sensor but it looks like I have to break the sealent that runs around the gas pipe that goes from the gas valve to the burners in order to remove the burner unit that will allow me to look at the sensor and igniters. Anyone have any ideas as to what is happening?
Thank you so much for the pictures. I knew what the problem was because we had the guy out a couple of years ago and he showed my husband where it was at. My husband is not due home for 4 more hours and the furnace wouldnt come on. You gave me great information and it worked the first time! Thanks again.