Example Furnace Flame sensorMy 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 regularly (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

sample gas furnace flame sensor

You can actually buy replacement flame sensors online. They’re very affordable and easy to install.

In most modern gas furnaces they’ve done away with the old pilot lights and gone to electronic ignitions which work similarly to gas grills: gas starts flowing and an electric spark is used to light the gas. If you’ve ever heard your gas heat “clicking” a few times before starting up that’s the electric started firing a spark in front of flow of natural gas in an effort to light a flame.

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!), but often they’re just corroded or need to be scratched clean a little bit to keep operating well. I don’t suggested trying to replace the flame sensor in your own furnace unless you’re pretty confident about what you’re doing.

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!