How To Replace Furnace Flame Sensor – Step-By-Step Guide

Furnace flame sensors go bad quite often. Fortunately, it’s just dirt buildup that you can remove with simple wiping most of the time.

HAVC experts recommend using a ball of soft steel wool to wipe or softly scrub the dirty flame sensor to remove the dirt. With that done, your sensor should begin to work normally again.

However, what if cleaning furnace flame sensor doesn’t fix the problem? What if it’s a bigger issue that doesn’t resolve no matter how you scrub the sensor? In that case, you may need to replace the flame sensor. 

Read on to find out common signs that you have a bad flame sensor. We also explain how to test the flame sensor to determine if the problem is fixable and the process of replacing a furnace flame sensor.   

What is a Furnace Flame Sensor?

The furnace flame sensor is a safety device found in all gas furnaces. Its sole purpose is to determine whether there’s a flame on the burner and communicate the feedback to the circuit board. The circuit board then uses the information to determine whether to keep the gas supply ON. 

If there’s no flame on the burner(s), the sensor will send a “no flame” feedback to the circuit board, which in turn cuts the gas supply, effectively turning off the burner.

However, when there’s a flame, it will send a “flame present” feedback. In this case, the control board will instruct the furnace to keep the gas supply to the burner(s) open to keep the flame burning. 

The flame sensor itself is a thin piece of wire with a porcelain base that serves as insulation. It sits right outside the burner assembly. The flame sensor may be straight or bend at 45 degrees or 90 degrees, depending on the furnace model. 

The thin piece of wire at the tip usually dips into the flame area above the burner, where it instantly senses fire upon furnace ignition. Meanwhile, the porcelain base connects to the burner assembly.  

How to Tell if Your Furnace Flame Sensor is Bad

If you’re having problems igniting the furnace or keeping it ON, you might have a problem with the furnace flame sensor. The following are a few signs that it may indeed be a bad furnace flame sensor;

The Furnace Lights Up Then Goes Off 

Many things can cause the burner flame to go off. So, you want to pay attention to a few additional signs before you lay blame on your flame sensor. 

First, if it’s a flame sensor issue, the burner will go off within 10 seconds after coming on. That’s because the furnace only waits for the flame for ten seconds after ignition. If the flame isn’t seen after this period, it cuts off the gas supply, effectively shutting the heating system. 

Additionally, this issue happens a total of three times before the heater goes into “lockout.” Lockout happens when the furnace permanently stops itself, such that it doesn’t come on no matter how hard you try – until you fix the underlying issue. 

Also, read how to fix a yellow flame on gas furnace

The Flame Sensor Base (Porcelain) is Cracked

The porcelain at the base of the flame sensor serves as insulation. It also shields the wires within the base from damage. Therefore, a cracked base is usually the first sign of a compromised flame sensor.

Commonly, it means that the base can no longer hold the bimetallic wires apart to maintain the voltage potential. The result is short-circuiting, an unintended connecting of the internal components. 

The Sensor is Corroded and has a Buildup of Soot

One look at the flame sensor is often enough to tell you that it may be compromised. As long as the unit is badly corroded and has a buildup of soot or dirt, it would be best if you did something about it. 

Why? Because dirt and soot negatively impact the flame sensor’s sensitivity (capacity to detect a flame). The same applies when the sensor is corroded.

As a result, it may not detect the flame or may only “see” a small flame even when you have a healthy flame on the burner. In both cases, the circuit board would cut the gas supply, effectively shutting the furnace. 

What Causes a Furnace Flame Sensor to Fail?

The furnace flame sensor can fail for many reasons. However, in most cases is one or more of the following;

  • Incorrect position: Maybe the sensor is positioned such that it doesn’t “feel” the flame when you ignite the furnace.
  • Dirt buildup: A dirty flame sensor can fail to capture the strength of the flame. It may also cause the wire end to bend. 
  • Short-circuiting: If the sensor short circuits, it will lose its ability to conduct electricity. So, it can longer detect a flame. 

What Happens When a Furnace Flame Sensor Goes Bad?

When the furnace flame sensor goes bad, you’ll notice a few things. Typically, the furnace will not come on, no matter how hard you try. It will come on and go off repeatedly, then get to a point where it locks out and refuses to come on completely. 

When Should I Replace my Furnace Flame Sensor?

You might have come across a few blogs saying you should replace your furnace flame sensor every 2-3 months. While that isn’t a bad practice, the best solution is only to replace a defective flame sensor, i.e., when it refuses to work. 

How to Replace a Furnace Flame Sensor Step-by-Step

Fortunately, replacing the furnace flame sensor isn’t as difficult as you might imagine. It’s also completely safe as long as you turn off the furnace before you begin. 

You’ll need;

  • An open-ended wrench 
  • Screwdriver or nut driver
  • New flame sensor

Here’s how to proceed;

Step 1: Turn Off the Electrical Power

This is very important as opening the furnace while it’s switched ON can cause electrocution. To switch off the furnace, toggle the switch mounted on the furnace to the OFF position. If it’s not on the furnace, check the wall next to the furnace. 

Step 2: Turn Off Gas to the Furnace

Locate the valve handle on the gas pipe running into the furnace and ensure it’s perpendicular to the pipe. If it’s parallel to the pipe, it’s ON and supplying gas. 

Step 3: Open the Furnace’s Access Cover

If the furnace has been running, wait at least 30 minutes for it to cool down. Then remove the compartment door to access the burner area. Depending on the furnace type, you may need to unlock twist locks on the compartment door or unscrew a few screws. Meanwhile, for others, you need to lift the door and pull the bottom outwards to remove the door. 

Step 4: Locate the Flame Sensor

The flame sensor is located right next to the burner. You can locate it by tracing the pipe that runs from the gas valve upwards, in which case you’ll find it on the last burner. Alternatively, look for a thin metal piece with a white porcelain base. 

Step 5: Remove the Old Flame Sensor

The procedure here will also depend on the type of furnace. In many cases, you need to detach the old flame sensor electrical lead. However, if the flame sensor has a permanently attached lead wire, detach it from the sensor’s base. 

Additionally, many flame sensors are held in place by one or two screws which you must unscrew to remove the sensor

Step 6: Compare the Two Flame Sensors 

Flame sensors are extremely sensitive to the extent that your furnace may refuse to work if you use the wrong one. So, before you install the replacement, compare it with the old sensor to find out if you have the right sensor. If so, proceed. Otherwise, get the right sensor. It’s best to get an OEM sensor. 

Step 7: Install the New Flame Sensor 

Many flame sensors come with a gasket for installation. Whichever the case, mount the new sensor in place, making sure that that thin wire at the tip is in the path of the burner flame. Then reattach the electrical lead and screw the unit in place. 

Step 8: Return the Furnace Door

Depending on how you opened the door, return it and screw it in place or twist the lock as appropriate. 

Step 8: Turn On Gas and Electricity

Starting with the gas supply, turn the lever so that it’s parallel with the gas pipe. Then toggle the electrical switch on the furnace. Now, you’re ready to use your furnace. 

How Much Does It Cost to Replace a Furnace Flame Sensor?

The cost of replacing a furnace flame sensor varies from one furnace to another and the service charge. It also depends on whether you opt for an Original Equipment Manufacturer (OEM) replacement or universal sensor. 

Generally, though, a replacement flame sensor costs $6 to $75, with most units priced between $20 and $40. OEM replacements are typically more expensive, costing $30+. 

Beyond the cost of the replacement sensor, you need to consider service charges. You can avoid this cost by opting for a DIY replacement.

However, homeowners choose professional servicing to avoid potential issues down the line. An HVAC professional will charge you $60 to $150 for flame sensor replacement, depending on the type of furnace and your location. 

Therefore, the total cost to replace a furnace flame sensor ranges from around $65 to $225, though it can be slightly lower or higher. 

Flame sensor replacement is a straightforward process to handle yourself as long as you have basic HVAC skills. However, it’s even better to let an expert handle it.