What To Do With A Dirty Flame Sensor

What To Do With A Dirty Flame Sensor

It can happen to just about anyone. Your furnace, which you count on to keep you warm in the coldest months of the year and on below-freezing nights, starts to rapidly cycle on and off. Perhaps, instead, it tries to kick on but doesn’t run for more than a few seconds before it suddenly clicks off, pilot and all. This can be a panic-inducing situation. After all, nobody looks forward to the thousands of dollars it can cost to get a new furnace.

There’s potential good news. The fix may not only be inexpensive, but also might be one you can diagnose on your own. The vast majority of furnace calls professional HVAC technicians receive of this kind involve a simple, basic flame sensor cleaning. Let’s learn about what a dirty flame sensor is, what you can do with it and how cleaning it can result in your furnace working just like a new piece of equipment.


A furnace flame sensor is a very basic part of your furnace. It’s located on the burner assembly and is just a thin metallic rod. It’s usually bent, and it sits just in front of the pilot flame inside the furnace. When the thermostat tells the furnace to kick on, the gas valve opens up and fires gas into the combustion chamber, where the pilot light or ignitor lights the burners.

The sensor is there to confirm that there’s actually a fire present when the gas is open. Without this, your pilot light could go out and fill your home with dangerous gas. This gas can build up and be deadly for many reasons, from gas poisoning to the potential for explosions.

The flame sensor, then, is a safety measure. If it doesn’t detect a flame when the gas is on, it automatically shuts everything down to make sure you’re safe. On most modern units, this shutdown can happen up to three times before the system goes into a lockout and sits for an hour before it tries again. This happens to let any gas buildup dissipate.

Unfortunately, if there is a flame and the sensor simply isn’t detecting it, this can result in being stuck without heat for an hour, and even worse, can create wear and tear on other parts in your furnace. This will reduce efficiency and can result in more expensive fixes.


People tend to use the terms “flame sensor” and “thermocouple” interchangeably. In truth, while they basically perform the same job, they’re not the same thing. If your furnace uses an electronic controller, it’s got a flame sensor. Thermocouples are used on standing pilot furnaces that have pilot lights. The thermocouple sits near the pilot light and gauges whether it’s working properly, while the flame sensor detects that the burners are lit once they start to emit gas and the igniters light them up.


Very often, the problem isn’t a bad part or faulty equipment. It’s simply a corroded or dirty flame sensor. Because your sensor spends so much time immersed in flame, it has a tendency to collect carbon deposits, dust and other particulates which can burn onto the sensor. These are very sensitive wires which have to detect fire in a very short window of time.

What this means is that the slightest collections or deposits on the sensor can cause it to fail. Fortunately, it’s often a very easy process to take care of the issue.


A number of signs that your flame sensor is fouled, corroded or otherwise bad are easily detected. First, check for your gas burners to light up and then go out after a few seconds. This is the most common sign of a sensor issue. Second, if you can actual see soot covering the tip of the sensor, it’s time for a cleaning. If you look at the tip and it’s black, it’s time for service. Finally, if you see cracked porcelain on the sensor casing, you’ll need a replacement. The porcelain is an important insulator that stops the sensor from being grounded on the bracket of the furnace.


As with most maintenance issues, cleaning a furnace flame sensor is a multi-step process that’s not difficult but can be exacting. If you’re not sure of any aspect of it, it’s probably a good idea to call on the professionals. Still, it’s always helpful to have an understanding of what’s involved, and a little knowledge can go a long way.

Generally speaking, the process involves turning off the furnace, removing the sensor, cleaning it off, replacing it and double-checking the results. Let’s break down each step in a little more detail.


The first key factor, turning off the furnace, is the most important step in the entire process. This is because attempting to work on a furnace while the gas is on can be very dangerous. Not only can it result in gas buildup, but if the furnace should kick on while you’re working, you can get a severe burn or even create a fire hazard in your home. Usually, turning off the furnace is as simple as flicking a toggle switch that will look like a light switch. If you don’t have one of these, turn it off at your home’s breaker box.

If your furnace doesn’t use an electronic control, you may have to shut off the gas valve to the system, as well. Again, if any of this is confusing, you’ll want to contact a professional for help.


Next, remove the old sensor from the furnace. It’s usually pretty easily accessible and is mounted by a quarter-inch hex-head screw. Remove this screw and the flame sensor should easily slide out so you can clean it. You may have to detach a wire from the end of it if you need more room to work, but often, this isn’t necessary to do.


Next, get a very light grain sandpaper, a wire brush, or steel wool. Use this to rub the metal rod gently, and only the metal rod, to clean the dirty flame sensor. You’re not sanding down a rusty car, here; you’re just trying to get rid of grit and buildup on the sensor. Be gentle, and don’t go overboard. When you’ve got the grit removed, use a soft cloth or paper towel to wipe it down.


After you’ve cleaned the sensor, simply reconnect it the same way you disconnected it. If you disconnected the wire, reconnect that. Then slide the sensor back into its housing and re-mount it with the hex screw. Replace the doors of the furnace and kick the power back on.

It could take a few extra seconds for the unit to start up than you’re used to — that’s okay and is normal. Sometimes the fan will kick on and just run a bit. It’s likely the unit is running through the cycles and checks before it fires up again. After it goes through the system reset, it should operate normally from your thermostat control. Be sure that it fires up and runs properly.


Replacing a broken sensor is basically the same process, except you’ll need to buy a new sensor and put it in rather than clean the old one. It may also be a good idea, however, to call in a professional HVAC technician to diagnose the problem and be sure of how to fix it.


If you have issues with a dirty flame sensor, call the professionals to get the help you need. Don’t worry if it seems confusing. Legacy Air can take care of all of your furnace flame sensor issues and any other furnace problems you might have. Give us a call for more information or to schedule your next appointment today.

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