How To Clean A Furnace Flame Sensor? (5 Step Guide)

A dirty flame sensor is one of the most common issues that homeowners encounter with their furnace systems.

If you find that your furnace fires up, but shuts down shortly after, the chances are you’ve got a dirty sensor, too.

Fortunately, it’s a super simple fix, and one that most people, even those without mechanical inclination, can tackle themselves, without the need for a costly HVAC repair service.

In this proHVACinfo guide, we’ll be learning about:

  • What You Need To Know About Furnace Flame Sensors (why they are causing problems)
  • Tools And Supplies You’ll Need For Cleaning A Furnace Flame Sensor (don’t get started without these 3 things!)
  • How To Clean A Furnace Flame Sensor (5 step guide)

And much more!

What You Need To Know About Furnace Flame Sensors

Gas powered furnaces are extremely safe and efficient heating systems, but absent safety features like flame detectors, they can become ticking time bombs, literally.

The flame sensor is a slim metal rod mounted in porcelain , placed in front of the flame jet inside the system.

They conduct electrical current, which detects the presence of a flame due to an increase in temperature.

The flame sensor’s job is to detect the presence of a flame in the furnace if the gas valve is in the open position and the gas is flowing.

If the flame has been somehow extinguished, and there is no automation telling the gas valve to close, this can cause an extremely dangerous explosion risk.

To prevent an accumulation of gas in the system, or worse, from leaking out into your home, the flame sensor has to act almost immediately upon ignition of the burners.

As a safety measure, if no flame is detected, the system will automatically shut off the gas flow.

Most residential systems have a lockout period if multiple attempts are made to start the furnace with a faulty flame sensor.

This safety automation will stop the system from operating altogether for a predetermined period if the maximum number of starting attempts are exceeded in a certain window. The number of attempts and period of the lockout window vary between systems and manufacturers.

This is why a faulty flame sensor can cause your furnace to start up and shut down shortly thereafter.

Because furnace flame sensors are such simple parts, all out component failure is pretty rare. In most cases, exposure to open flames leads to a buildup of carbon on the sensor itself.

As the sensor works by indicating a change in temperature, even a microscopic buildup of carbon can alter the temperature at which the sensor detects a rise in heat.

The greater the carbon buildup, the more dramatic this will be, and in turn, the longer it will take to acknowledge the presence of a flame.

If the period of time the sensor takes to detect the heat increase exceeds the built-in tolerances, the system will shut down because it believes that no flame is present.

In addition to carbon build-up, flame sensors can also collect dirt and dust quite easily. Most people tend to keep their furnaces in their basements, which are typically a little more damp than the rest of their homes which will cause dirt and debris to stick to, and build up on components like your flame sensor.

Supplies You’ll Need For Cleaning A Furnace Flame Sensor

Because this is a task almost all homeowners will be able to accomplish on their own, the tools required are quite limited. To access, remove, and reinstall a furnace flame sensor in most residential systems, you will need:

  1. A hex driver or socket set – The majority of furnace systems will only need a ¼ inch hex driver or socket, but as this varies between manufacturers, having an assortment is helpful. We always find it useful to have a ratchet set, as it speeds up the panel removal and reinstallation.
  1. Wire wool and light grit sandpaper – Wire wool and light grit sandpaper are both essential abrasives for getting the carbon build-up off the sensor.
  1. Microfiber rags – Microfiber rags are a better choice than paper towel or non-microfiber rags because they don’t leave strands or fibers behind when you’re cleaning. If you wipe down the sensor with paper towel, and you leave paper fibers behind, they will burn off when the sensor goes back in the system, which will get the rebuild of carbon off to a head start – not something you want!

How To Clean A Furnace Flame Sensor (5 Steps)

Step 1: Shut Down the Power           

Whenever you dismantle a system like a furnace, there is a risk of electric shock, and not only that, if the furnace is not shut down, and it starts by a thermostat or timer while you’re working, you could get seriously burned.

Out of an abundance of caution, you should always remove the power supply to the unit.

Remember, simply stopping the automatic temperature control at the thermostat does not count as shutting off the system! You need to physically shut off the power.

Most systems have a hard on/off switch somewhere on the unit. If you can’t find one, identify the correct circuit at your breaker box and switch it off. We like to suggest using the on/off switch and the circuit breaker if possible.

The method of operation for every manufacturer and system model is different. For that reason, the instructions here are only a rough guide to the overall process. Always read your manual, and if you’re not confident that you can take on this task safely, stop and call a professional.

Step 2: Remove The Flame Sensor

Using your hex driver or socket set, open the outer panel and store the screws somewhere safely nearby.

Your flame sensor should be easy to access, and is likely somewhere near the opening of the panel.

It should be mounted with the same hex screws as the outer panel screws, so you shouldn’t even need to change tools.

Even when unmounted, the flame sensor should still be attached by an electrical wire. If it’s attached by a quick connection, it’s probably worth removing it altogether in order to get the best access to all areas of the sensor. If not, you should probably have enough slack in the wire in order to clean the exposed metal part of the sensor.

Step 3: Start Cleaning The Sensor

Remember, the base is most likely made of porcelain, and is therefore quite delicate, so be conscious of this when handling your flame sensor.

It’s now time to start scrubbing! It’s only necessary to clean the metal rod, so avoid contacting the porcelain or other components with your cleaning tools.

Start gently rubbing the rod with the sandpaper in order to remove the bulk of the buildup. Once you see bare metal again, it’s time to slow down.

You don’t want to remove any material other than carbon buildup. Once the bulk of the carbon and other dirt is off, go in with the wire wool, especially around any curves on the rod.

When you’re happy with the state of the sensor, take your microfiber rag and wipe off any carbon dust left on the sensor and the sensor mount.

Step 4: Reassemble

Now that your flame sensor is as good as new, it’s time to put it all back together again. The process for reassembly is the exact reverse of the disassembly.

If you physically disconnected the sensor, now is the time to reattach before setting it back in its mount and screwing it tight.

Put the cover panel back in place and tighten the securing screws back up.

Don’t over tighten these, they should be a little squeeze past finger tight but not much more, or you risk stripping them.

Turn on the gas and power supplies, and you’re ready for the next step.

Step 5: Test The System

Modern furnaces are made with a number of built in safety controls, some of which may kick in automatically after the system has detected a restart after a period of no power.

These are normal, so don’t worry if your furnace doesn’t roar into life the second you turn it back on. After the system cycles through its checks, it should either be in standby, or start running, depending on the thermostat settings.

If it’s in standby, go to your thermostat and adjust the temperature so that the furnace should turn on 1 degree before the ambient temperature. This will check that the thermostat link is still working after your reassembly, and it should also test the flame sensor, too.

Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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