How To Clean A Flame Sensor On A Furnace

Dirty flame sensors are the primary cause of ignition failure in gas furnaces. A flame sensor covered in dirt or soot lacks the sensitivity to detect the presence of a burner flame, which in most cases, will keep the furnace from staying lit.

The problem is most common during ignition but may also manifest when the furnace is up and running. The moment the flame sensor loses “sight” of the flame, the heater (or boiler) will go off.

To this end, it helps to learn how to clean the flame sensor. Of course, you can always call the HVAC technician. However, the majority of professional technicians charge at least $60 for servicing.

It may be wiser to do it yourself and save the money and waiting time for a process that takes about two minutes or less and doesn’t require much HVAC know-how.

Below we discuss how to clean your furnace flame sensor step-by-step, as well as answer several common questions about flame sensor cleaning.

How Do I Know if my Flame Sensor is Dirty?

Unfortunately, it’s not as easy as you may wish. For example, sometimes, you may think you have a dirty flame sensor when the furnace has an entirely different problem.

Alternatively, you may think you think it’s a problem with a different part of the furnace, perhaps the pilot system, when in fact, you have a dirty flame sensor.

So, to know for sure that you have a dirty flame sensor and rule out other potential issues, you need to complete a two-part test.

Keep an Eye Out for Tell-Tale Signs

A bad flame sensor will usually exhibit a few standard signs, irrelevant of the type of furnace or fuel type;

  • Burner flame keeps going out

If you have a malfunctioned furnace flame sensor, the burner flame will struggle to stay lit. The pilot system will operate normally (assuming that the pilot sensor is in good condition). However, the burners will come on only briefly before going off. In most furnaces, the burner will stay lit for about 10 seconds before going off.

This may repeatedly happen until you give up, though most furnaces only allow three trials before the unit goes into lockout – more on this shortly.

The burner struggles to stay lit when you have a dirty flame sensor because a dirty flame sensor may not be able to detect the presence of a flame, even if there’s one. In addition, given that furnaces are designed to cut fuel supply and shut off the system if no flame is present (for safety reasons), the flame will not stay lit.

  • The furnace won’t come on at all.

We’ve mentioned that your furnace may go into lockout. Lockout is a state in which the furnace is unresponsive to any input. It doesn’t matter how many times you press the start button. If the unit is in “lockout,” it won’t respond – until you manually reset the furnace.

Most furnaces have two lockout modes – soft lock and hard lock. Soft lockouts happen when you start the furnace, but it doesn’t detect a flame within the set time (usually 10 seconds). Meanwhile, hard lockouts occur if you attempt to restart the furnace two more times after a soft lockout.

If your furnace enters a hard lockout, you must fix the underlying issue then manually reset the furnace. Otherwise, it won’t respond to your input attempts.

Physically Check the Sensor for Dirt Buildup 

If you experience one or both of the above issues, you want to physically inspect the flame sensor to find out if it’s dirty.

A physical inspection is important because the flame sensor can also become permanently damaged. For instance, an electrical fault within the burner system can short circuit the sensor, causing it to malfunction. We recommend checking for two things;

  • Dirt/soot/carbon buildup

If you know the basics of a flame sensor, you’ll be aware that it sits right beside the burner assembly, with the tip dipping into the flame area right above the burner. Due to the positioning, the flame sensor can accumulate a lot of smoke, soot, dirt, and even carbon.

If you’re wondering what carbon buildup looks like, it’s no different from soot. Soot is one of the byproducts of carbon residue.

Is the sensor, especially the wire tip that protrudes into the flame area, covered in a black residue? Can you see strings of smoky residue dangling from it? If so, then you have soot/carbon buildup. Sometimes the buildup is so intense that it covers even the base.

Besides soot and carbon, you may also find the flame sensor covered in a thin layer of dirt and debris. The dust usually comes from the air drawn from your home into the burner system to facilitate the combustion process.

  • Cracks on the porcelain base 

 This second part will tell you whether you need to replace rather than clean the flame sensor. Although you may also need to replace a dirty flame sensor if cleaning doesn’t resolve the problem, at least you can try to fix it first. However, if the porcelain base is cracked, don’t waste your time cleaning. The sensor is damaged and must be replaced.

Why? Because a crack on the flame sensor’s porcelain base is the surest sign of short-circuiting. A short circuit cannot conduct electricity anymore.

How to Clean the Furnace Flame Sensor – Step-by-Step

If your flame sensor is dirty, without cracks on the base, you can clean it to restore its function and prevent the furnace from going into lockout. You can also clean it before a manual reset if it goes into lockout. Follow the steps below;

Step 1: Switch off the power 

This is very important as working on the furnace while it’s ON causes the risk of electrocution. To switch off the furnace power, locate the toggle switch on it and ensure it’s OFF. If it’s not on the furnace, it is likely to be on the wall just beside the furnace.

Step 2: Switch off the gas supply 

The gas lever is located on the gas line leading to the furnace. It’s ON when parallel to the gas pipe and OFF when perpendicular to the line.

Step 3: Remove the sensor 

After confirming that the unit is cool-to-touch, remove the access cover to locate the flame sensor. You may need to unscrew a few screws. Once inside the burner area, the flame sensor is a thin metal rod with a white porcelain base. Unscrew it and pull it out gently. If it’s hooked to the control board, detach the lead wire at the sensor’s base. Then pull it out.

Step 4: Clean the sensor

Gently rub the metal rod (only the rod, not the base) with a ball of soft steel wool. You can also use a soft scotch brie-type pad. Never use sandpaper as it can damage the flame sensor rod. Once you’ve loosened the dirt, use a clean paper towel to wipe clean the dust left behind.

Step 5: Replace the sensor

Begin by reconnecting the lead wire that you took off. Then remount the flame sensor to the burner assembly and screw it gently but tightly using a ¼ inch screw. From there, replace the access door and turn back the gas supply and electric power.

Step 6: Check your results

Turn the furnace ON to see if the burner stays lit. If the cleaning fixes the issue, the burner will stay lit with no further issues. If it doesn’t, try again. If the second trial fails too, it may be time to replace the flame sensor.


What Causes a Dirty Flame Sensor?

A dirty flame sensor can result from many issues. However, the two leading causes are dirt (dust and debris) and soot/carbon buildup.

Can You Clean a Flame Sensor?

Yes, you can clean a flame sensor. Indeed, you can do it yourself (DIY) as long as you remove the furnace access door to locate and detach the sensor. However, if you’re not so confident in your DIY skills, consider hiring a pro.

Why Do You Need to Clean Furnace Flame Sensor? 

You should clean your flame sensor because a dirty sensor can cause furnace dysfunction. The furnace will keep going off within seconds of ignition and may even enter lockout mode, thus requiring a manual reset. Manual reset costs a few hundred more if you hire a professional.

What Happens if You Don’t Clean the Furnace Flame Sensor?

If you don’t clean the furnace flame sensor, you will experience dirt and soot buildup. Often, this can cause the furnace to refuse to start – no matter how many times you try. When this happens, you must clean or replace the sensor to use the furnace.  

How Often Should You Clean the Furnace Flame Sensor?

You should only clean the flame sensor during the scheduled annual maintenance or when you run into a sensor issue. A few HVAC professionals recommend cleaning every 2-3 months. But we believe this is unnecessary as a good flame sensor can last the entire heating season after pre-season cleaning.

How Much Does it Cost to Clean the Furnace Flame Sensor?

The cost to clean a furnace flame sensor depends on several factors, including the type of furnace and your location. Nevertheless, many HVAC professionals charge the standard service fee, i.e., $60 to $150 for flame sensor cleaning. Remember that you can avoid the cost if you clean it yourself.

What Can I Use the Clean the Furnace Flame Sensor?

The best answer here is to follow the manufacturer’s guidelines. However, if you can’t find your user guide, we recommend cleaning with a soft steel wool or scotch brie pad. We’ve seen a few people recommending sandpaper. However, we don’t encourage it as sandpaper can damage the sensor.

That’s all. Hopefully, you now know how to clean the furnace flame sensor yourself. Don’t hesitate to call the experts if you’re not confident in your DIY skills.