Fixing a Common Gas Furnace Problem: Flame Sensor Cleaning

While there are a lot of parts working inside your heating system, one of the most common reasons for a furnace breaking down is because of one particular part – the flame sensor. The main indicator you have a problem is finding out your furnace will not stay lit.

Standard Plumbing, Heating & Air Conditioning has been repairing furnaces in the Manhattan, KS area for nearly 100 years. So you can imagine that we have seen just about every different kind of heating problem there is.  Often a system may seem completely broken, but actually, it can be easily fixed.

Sometimes we respond to a call from a homeowner who tells us that, up until recently, their furnace was working fine but not the furnace will not stay lit. They could hear the burners come on, but pretty quickly the furnace shuts off. This can be very confusing for a homeowner, but it’s often a problem that we can easily solve. Not only that, but it may also be one you can solve yourself!

This common furnace problem is a dirty flame sensor.

Clean furnace flame sensor in hand

What is a Flame Sensor? And How Does a Flame Sensor Work?

A furnace flame sensor is a stainless-steel rod with a ceramic or porcelain cover. You’ll find it behind the burners, most typically on the opposite side of the ignitor. Simply put, its job is to sense when there is a flame. When there is, the flame sensor signals for your furnace to keep sending gas to keep the heat going.

Does every furnace have one?

No. If yours has a spark ignitor instead of a sensor rod, then the furnace ignitor is also doing the job of a sensor.

How does a flame sensor work?

Your furnace has a control board that sends electricity to the flame sensor, which then carries the flame to the burner face. The sensor lets the control board know that the current is flowing. That tells the control board to continue allowing gas to flow, too.

If the sensor doesn’t tell the control board that the current is going where it should, then the gas valve closes, and the heat stops coming through your vents.

Flame sensor near burners inside a furnace

Furnace Will  Not Stay Lit? DIY Steps for a Testing for a Dirty Flame Sensor

It is natural for a furnace sensor rod to become dirty over time. But if it gets too dirty, it could stop doing its job. The control board will not be able to sense the current is flowing if the sensor is covered in too much build up.

Fortunately, is it quite easy to clean a flame sensor. Most sensors are easily accessible and so a homeowner may want to clean it themselves.

Scotch-Brite™ pad to be used to clean a dirty flame sensorHOW TO CLEAN A FLAME SENSOR

    1. Turn off the furnace power switch.
    2. Remove the sensor rod. It should be easy to remove with a screwdriver. Do not use a wrench, as that could cause the sensor’s cover to crack.
    3. Using a Scotch-Brite™ pad, rub the sensor rod to clean it of all dirt and gunk. Do not use anything harsh like sandpaper.


    • You can use a dollar bill to clean it. Just fold the bill over the rod and, while tightly gripping it, rub it back and forth until the rod becomes somewhat shiny.
    • Do not touch the flame sensor with your fingers. The oils on your hands can get on the sensor and make it more prone to get dirty later.

Furnace Shutdown

Having a dirty flame sensor causing your furnace to stop working can mean you are facing an additional issue. A dirty sensor that is causing the ignition to go out over and over again may eventually cause the furnace to stop working. This is likely just a temporary shutdown, but one that will need to be dealt with in order to get it up and running again soon.

This shut down happens with furnaces that have particular safety function. If a furnace control board senses ignition attempts have failed 5 times in a row, it may shut down the furnace automatically and stay off for a few hours.

There is a work-around to waiting several hours. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. To do this, turn off the furnace’s power switch and wait about 10 seconds before turning it back on.

When to Replace the Sensor

It is not likely a flame sensor needs to be replaced. You should be able to sufficiently clean a sensor no matter how dirty it is. But they are not expensive parts, and you can choose to replace it instead of cleaning it. There are different shapes of flame sensors so be sure you are purchasing the proper kind.

Baxter straight flame sensor rod Rheem curved flame sensor

What if Cleaning the Flame Sensor Doesn’t Fix the Problem?

If after cleaning the dirty sensor the furnace still won’t stay lit, you may find that the burner face is dirty.

To clean the burner face, remove the front plate of the burners with a screwdriver. You will want to be careful to not bump the ignitor. Ignitors are fragile and can be damaged from the oils on your hands.

Individual burners typically pop out easily. You’ll likely find the burner closest to the flame sensor is the dirtiest. If the burner’s base is dirty or corroded, clean it with sandpaper, steel wool or Scotch-Brite pad.

Potential Electronic Issues

Cleaning a flame sensor or burners will not get the furnace running again if the problem is an electronic one. A wire may simply be loose or broken, keeping the electric current from getting to the sensor. A trained and experienced HVAC technician will be able to detect whatever the problem is and fix it.

Standard provides emergency furnace repair serviceContact Standard Plumbing Heating & Air Conditioning to handle whatever furnace repair or service you need.

With a team full of qualified heating repair technicians, we can repair and service any heating system or unit. We provide service to homeowners and businesses in Manhattan, Junction City, Wamego and surrounding Kansas towns.

You can rely on our team of highly trained and experienced technicians to get the job done quickly, professionally, and safely. Call us at (785) 776-5012. We are available for emergency furnace repairs 24/7.