Dirty Flame Sensor? Should I Clean it and How?

Dirty Flame Sensor? Should I Clean it and How?

Yes, you should clean your furnace flame sensor if it isn’t doing its job – and shutting down your furnace – or as a part of preventative maintenance.

Cleaning a flame sensor is one of the easier furnace repair tasks, and this short Pick HVAC furnace how-to guide gives flame sensor cleaning step by step instructions.

This page walks through finding the flame sensor, removing it, inspecting it for dirt and damage, cleaning it and re-installing it. After the how-to flame sensor information, troubleshooting the flame sensor is discussed, so you can determine if it’s the problem. And if the flame sensor is damaged and needs replacement, that topic is covered too.

Did you know? This article could also be called Dirty Thermocouple? Should I Clean it and How? Yes, the terms flame sensor and a thermocouple refer to the same part on a gas furnace.

Use the Navigation to go to the section more relevant to your research today.

First Stuff

It is recommended that you turn off power to the furnace either at the circuit breaker panel or, just as good, using the switch on or near the furnace. It looks like a standard light switch. Some like to turn off the gas too for an extra measure of safety.

How to Clean a Furnace Flame Sensor

Here is the step-by-step flame sensor cleaning checklist. Details for each section follow.

  1. Turn off power to the furnace
  2. Find the flame sensor – wires attached, sticking through the inner furnace wall into the combustion box
  3. Remove the flame sensor by taking out the one or two screws
  4. Inspect the flame sensor ceramic base and probe for damage – and if you find a crack, jump down to Flame Sensor Replacement
  5. Clean the probe with Emery cloth or similar material
  6. Re-install and fasten the flame sensor
  7. Restart the furnace

How to Find the Flame Sensor

It’s where the flames are : )  So, you’ve got to remove the cover to the furnace front using the screws, clips or twist knob – it varies by furnace brand.

Once the cover is off, find the gas line or the tubes where gas enters to be ignited. Just above or below the gas line and tubes, you’ll see a couple wires attached to something in the sheet metal, a device held by a single screw in most cases. That’s the flame sensor. To be sure, with plenty of light, see if there’s a probe-like end inside, on the other side of the sheet metal.

It looks like this when installed.

It looks like this when not installed. The probe might be straight or have an angle. The entire assembly includes the wires too.

OK – you got it? Now for the next step.

Remove the Flame Sensor

Most have one screw; a few have two. If you’re guessing, try a 1/4″ socket or nut driver first.

Remove the fastener(s) and place them right in front of you on a level spot inside the furnace or right next to it.

Gently grasp the base of the sensor, the part on your side of the sheet metal, and pull it out.

Some folks like to remove the wires before removing the screw, and go for it if that sounds better to you. Grasp the clamp gently with standard or needle-nose pliers, and gently pull it off.

Inspect the Flame Sensor

A dirty flame sensor is easy to clean. But if it is broken, cleaning it won’t help.

Check the ceramic base of the sensor, the place the probe comes out of. Look closely. Do you see a crack in the ceramic? The gas in your furnace burns at more than 3,000 degrees Fahrenheit. The probe gets very hot, and then it cools. When that happens, and the potential expansion and contraction that goes with it, the ceramic might crack.

Term Tip: The ceramic base of this device is called the insulator.

What about the probe? It is a solid piece of metal, but if it is cracked or excessively corroded rather than just dirty, it might not properly sense flame, and shut down your furnace as a result.

A damaged probe should be replaced – and you can certainly DIY. Flame sensor cost is about $8 to $50 for the probe or the entire assembly including the wiring. Cost and what kind you need are discussed shortly.

Clean the Flame Sensor

What you’re looking for in terms of dirt is discoloration or anything clinging to the sensor. It might also have rust or other corrosion since water is a byproduct of gas combustion and your furnace likely sits in a place like the basement or attic that can be humid in non-heating seasons of the year.

Typically, combustion in the furnace causes a little bit of soot and other carbon deposits that can stick to the probe and insulate it from “feeling” the heat of the burner. If it doesn’t get hot, it won’t give the “good to go” signal to the furnace, and the control board will shut off the gas valve. No gas, no flame, no heat.

Pick your cleaner. Aluminum oxide Emery cloth and sandpaper are flexible and wrap easily around the probe. Fine-grit sandpaper for wood is the second-best choices. Steel wool or a small wire brush are acceptable.

Tip: You want to remove the carbon buildup on the probe – not any of the probe’s metal. So, take ‘er easy. Rub gently patiently, and that should remove the carbon. If not, be a little firmer. Try not to score the metal. It’s not a big deal, but do the best you can to clean the flame sensor without damaging it.

Re-install the Sensor

Just do the opposite of what you did to remove it. Gently push the probe back through the hole. Install the fastener, and tighten it until it is snug.

That’s it.

How do I know the probe is pointing in the right direction? That’s a common question when the probe is angled. The beauty of it is that it can only go one way once the single fastener is replaced. If there are two fasteners and the probe is angled, re-install the sensor so that the probe is aimed at the flames – or the tubes/ports where the flames are when the furnace fires.

Replacing a Damaged Flame Sensor

What sensor should I buy? How much does a furnace flame sensor cost?

Most furnace flame sensors are universal – they’ll work on any gas furnace. However, you might be able to find one for the brand of furnace you have. A quick search shows a few Rheem gas furnace flame sensors. But here’s inside information – Rheem is one of those brands that makes universal parts that can be used in other brands. So, a Rheem flame sensor will work for Trane, Carrier, Goodman, Armstrong…a long list of furnace brands.

Key: Choose a sensor that looks like the one you’re replacing. If it’s straight, buy one with a straight probe. If angled 45-degrees or 90-degrees, go that route.

Flame sensor cost starts at about $8 for a universal unit with just the sensor. No wiring harness. The most expensive sensor-only products cost about $40.

If you prefer to replace the entire assembly including the wiring harness, expect to pay $25 to $75 based on the brand you select. You can find them for $100 or more, but shop around. Some sites price gouge.

Troubleshoot a Flame Sensor

We’ve created a page called How to Tell if a Flame Sensor is Bad – and How to Replace a Bad Flame Sensor. We summarize the information there.

The flame sensor must sense significant heat or it will signal the control board to shut down the process. Here’s what happens when the flame sensor doesn’t sense heat:

  1. The draft motor starts. You might hear it hum. 
  2. The hot surface igniter energizes and glows red.
  3. The gas valve opens to release gas, and you might hear the whoosh, the burner fires.
  4. The gas valve suddenly shuts, and the flame goes out. This is called a soft lockout because you can try to restart the furnace again after this. Some furnaces automatically attempt to restart, going through steps 1-3, and possibly 4, again. 

Starting a Furnace After a Hard Lockout

It’s pretty simple. If you’ve determined that the flame sensor is causing the lockout, you obviously will want to clean it or replace it before resetting the furnace.

  1. Turn off power to the furnace at the furnace or at the electrical panel.
  2. Wait one minute (read your manual – some furnaces can’t be re-started for an hour or more).
  3. Turn the power on.
  4. If the thermostat is set to call for heat, the furnace should start. If not, adjust the thermostat to call for heat.

If the furnace doesn’t fire, stay lit and warm your home, use the Free Local Quotes tab or the toll-free number on this page to get advice and service, if you want it, from licensed, pre-screened furnace repair contractors in your area. There is no obligation. 

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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