How to Bypass the Flame Sensor for Troubleshooting?

A question we often get goes something like, “is there a way to bypass / trick / jump out the flame sensor for troubleshooting?” Or “how do I bypass the flame sensor temporarily?”

In other words, how do you know if the flame sensor is faulty, reading “no flame” when a flame is present, and as a result shutting down the furnace?

Fact: Flame sensor issues account for more service calls than any other issue for most HVAC techs.

In this Pick HVAC furnace troubleshooting guide, you’ll discover whether it is possible to bypass the flame sensor, how to clean it, how to replace it, what a new one costs DIY or Pro and even where the flame sensor is located.

How Do I Bypass a Flame Sensor Temporarily?

Before you click away to another search result, let us assure you that what you’ll read here is correct – exactly what you’ll find elsewhere, only written more clearly and with more useful troubleshooting and workable solutions.

Well, while disappointing to some readers, here is the answer: No, bypassing the flame sensor isn’t a solution.

You won’t find steps to bypass the flame sensor anywhere. While some furnace technicians claim to know how to do it on some furnaces (newer furnaces won’t allow it), there is no commonly understood way to do it. “If I knew how to do it, I wouldn’t tell you for liability reasons” is a common response.

Here’s what one veteran HVAC pro says about it, “Bypassing safety controls [like the flame sensor], even when possible, doesn't prove anything. Use meters and instrumentation to troubleshoot properly.”

If your flame sensor has gone bad and is not working properly, you may think a temporary fix would be to bypass the sensor. Often when dealing with mechanical appliances there is a way to trick or jump around the problem. In theory, this might work, and you would get the furnace running by somehow bypassing the flame sensor.  Attempting to bypass the flame sensor is not a good idea, even if it were possible. The flame sensor is an important safety feature on your furnace.

Options Covered - Clean or Replace

OK, sorry about that answer. We hate to give it. If you read Pick HVAC for other equipment issues, you know that if there is a way to DIY, we give clear information on how to do it. It’s just not doable for flame sensor troubleshooting.

But what we do cover in this post will definitely help:  - As noted, related information including the cost of flame sensor replacement whether you DIY ($) or hire a technician ($$-$$$).

Switching out the flame sensor is good DIY preventative maintenance anyway, since the part is cheap - $6 to $75 depending on the brand and whether you buy just the sensor or the entire assembly. Most are less than $40. The content below explains your parts options.

Note: This is a Pick HVAC How-to post – one of many we’ve produced to assist homeowners in taking care of their own HVAC equipment issues or at minimum understanding the problem, so that they can discuss it with an HVAC technician. When an HVAC pro knows that you know what you’re talking about, it keeps them honest (most are honest anyway) in the information they share with you about diagnosis and repair. OK, here we go.

How Do I Know if a Flame Sensor Goes Bad?

If it ain’t broke, don’t fix it – That’s one approach, though again, replacement isn’t a bad idea.

There 3 telltale signs that your flame sensor is not working properly:

  • The furnace lights but turns off pretty quickly because the flame sensor isn’t “feeling the heat”
  • The flame sensor’s porcelain is cracked or broken – a visual inspection will reveal this, though you might have to unbolt it and remove it
  • There is heavy soot on the flame sensor, or it is corroded

Another possible sign that your flame sensor needs to be either cleaned or replaced is that your furnace will show an error code.  Most often the error code is a blinking or solid light on your furnace.  Codes can be numbers, a combination of numbers and letters or a series of blinking lights. Sometimes when the flame sensor is bad, the error code will read “LOCKOUT.”

How to Clean a Flame Sensor

If the flame sensor on your furnace has stopped working properly and you are relatively mechanically inclined, you can either clean or replace the sensor yourself. This will save you between $65 and $150, the average service fee to have a professional HVAC technician come to your home and either clean or replace the flame sensor. 

The following are the steps you should take properly clean the flame sensor:

  1. Shut off power and gas to the furnace
  2. Remove the mounting screw, and set it where you’ll be able to find it
  3. Remove the sensor
  4. Inspect it for cracks – and if cracks are present, move to the step-by-step flame sensor replacement section below
  5. Clean away soot and corrosion from the metal sensing stick using emery cloth
  6. Reinstall and fasten the flame sensor
  7. Re-power and start the furnace

How to Replace a Flame Sensor

If cleaning the flame sensor doesn't do the job and your furnace still will not stay on, then your next option is to replace the flame sensor. The proper steps to effectively replace a flame sensor are as follows:

If possible, identify the rating of the current flame sensor or take the part to the HVAC parts store or find one online that looks most like it

  1. Shut off the power and gas to the furnace
  2. Remove the mounting screw
  3. Pull out the sensor
  4. Detach the wire from the sensor
  5. Push the new sensor into the opening
  6. Secure it with the mounting screw
  7. Attach the wire to the new sensor, and make sure the connection is tight (you might want to gently squeeze it together with pliers)
  8. Restart the furnace

Where is the Flame Sensor Located?

The flame sensor is installed behind the access panel on your furnace.  You most likely will have to remove some screws or tabs to get the panel off. The sensor is a metal rod that has either white or yellow/brown porcelain around the base – and that might be all you see when you remove the panel. The flame sensor is mounted to the outside of the burner assembly and has a single wire attached to it. The sensor’s sensing stick is located inside the fire chamber/burn box. Some sensors are straight, and others have a 45-degree or 90-degree bend at the end. 

How Much Does it Cost to Replace a Flame Sensor?

Cheap fix – cleaning: Flame sensor replacement cost? Many times, if your flame sensor has gone bad you can simply clean the sensor with an emery cloth that will cost less than $5.

Replacement: Flame sensors can be purchased for anywhere between $6 and $75 online and from big box home improvement stores. Make sure that you check the furnace model and buy the correct sensor for your furnace.  Another option is to buy a universal sensor or purchase an OEM (original equipment manufacturer) part. See the section above to DIY the repair process.

If you decide that either cleaning or replacing your flame sensor is not something you want to do, then call a professional HVAC technician. There will be a minimum service fee of between $65 and $150 plus the actual cost for the flame sensor. If the flame sensor just needs to be thoroughly cleaned, then you will pay for the service fee – and once you see how easy the job is, you might DIY next time : )

Can a Furnace Work Without a Flame Sensor?

If you’re still wondering if there’s a trick that isn’t being shared, this might help.

A flame sensor is exactly what it sounds like.  It senses if there is a flame. If so, the furnace is working/gas is being burned. If there’s no flame, then you have a gas leak without a flame sensor.

As soon as your thermostat calls for heat, the furnace draft motor comes on, and in a few seconds the igniter heats up. The gas valve opens to release gas, and you’ve got flame. The flame sensor’s purpose is to sense the flame. This keeps the furnace firing correctly, and your home will be heated like it is supposed to. If the flame sensor does not work, then the furnace will automatically shut down and stop the flow of gas. If there were no flame sensor and no flame, gas would continue to flow without being burned, and that’s setting up the house for a major explosion.

So, to be completely blunt…..a furnace cannot work without a flame sensor. The flame sensor’s intended function is to keep you and your family safe, as well as your furnace working correctly. 

Further Research and Troubleshooting Information

We hope this information was useful, and in short order your furnace will be cranking out heat with a cleaned or replaced flame sensor.

If you replace the flame sensor, and the furnace still doesn’t cooperate, here are two options:

  • And see our Related Topics list in the right column for a wealth of additional information. 

DMCA.com Protection Status