My gas furnace is only a couple years old, so I wasn’t real happy when it 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…”

You can actually buy replacement flame sensors online.

We’d check the thermostat to see that the heat was set for 68 but the house was still only 66 degrees. So I’d turn the thermostat off, wait a few minutes, and then turn the heat on again. 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.

Each time I couldn’t get it going right away I panicked a little more until everything magically started running again. So I took a few safety precautions: turn off the heat, shut off the gas and took the cover off my furnace for a closer examination. You may also want to wait a few minutes before working on a furnace, just to let it cool down.

I started checking out my furnace a little more closely and I stumbled upon three little green LED lights that would give me error codes. Two lights were solid on when everything was working correctly, but one green light would blink whenever the heat stopped running. One the inside door of my gas furnace was a wiring diagram, a truncated table of error codes and a troubleshooting flow chart which was a little too technical for me to completely decipher. Most furnace companies don’t make their full technical manuals available to the public because it really isn’t safe for do-it-yourselfers to be rewiring gas furnaces. There’s just too much that could go wrong.

But as I was looking in my furnace, trying to understand how the whole system worked when I suddenly remembered having a similar problem with another gas furnace years ago when I lived in a condo. That time I called the technician out and he poked around a little bit and then pulled out some steel wool and quickly cleaned a little part. The furnace started working again almost immediately and he charged me $145.

You can see how difficult it is for me to get to my flame sensor.

That’s when I remembered it: the flame sensor! 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.

The flame sensor then checks to see how hot the flame is and helps the furnace regulate how much gas to use to keep things warm. The flame sensor is usually a 3 - 4 inch gray metal looking rod that has a little white porcelain base. It can be either straight or angled but it should be positioned right in front of one of the burners.

If this sensor doesn’t think the flame is hot enough it will simply put the furnace in a safety “lockout” mode because that could be the sign of a gas leak or other potential problem. This lockout mode could last anywhere between 15 - 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.

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.

Most people recommend gently rubbing some steel wool against a flame sensor to clean it, but mine is sort of hard to get to, so I just use a long screwdriver and gently scrape up and down and around the flame sensor as much as possible. You may or may not be able to see anything getting cleaner, depending on your situation.

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.

And, yep, I saved myself $145 in the process!