How to Tell If Flame Sensor is Bad

How to know if you have a bad flame sensor is quite easy to diagnose.

Signs of a bad furnace flame sensor are:

  • The furnace lights but then shuts down after a few seconds (short cycles)
  • The porcelain on the flame sensor is cracked
  • The flame sensor is sooty or corroded

This guide explains how to troubleshoot a bad flame sensor.

The Flame Sensor Purpose

This part does exactly what it says – senses flame to ensure the furnace has fired. If a gas valve is open but the furnace hasn’t fired, it isn’t just emitting raw natural gas or propane. It will leak out of the furnace and into your home where it can easily explode when a water heater fires up or a gas stove lights.

  • Good flame sensor: A properly functioning flame sensor will shut down the gas valve when it does not sense a flame when the furnace should be firing.
  • Bad flame sensor: When the furnace has fired and is burning hot, a bad flame sensor won’t sense the heat and will shut down the furnace.

Flame Sensor Location – How to Find a Flame Sensor

To locate the flame sensor, you will need to remove the furnace access cover. It might be held in place by tabs and slots, knobs or a few screws.

The flame sensor is a metal rod with white or dingy yellow/brown porcelain surrounding the mounting end. The porcelain insulates the sensor against grounding itself on the metal furnace frame. Some sensors are straight; some have a 45-degree or 90-degree bend near the end.

The sensor is mounted to the outside of the burner assembly. It has a single wire attached to it.

The flame sensor protrudes into an open fire chamber or through the housing into the interior where the flame is burning.

DIY Flame Sensor Diagnosis

Here is a step-by-step furnace flame sensor repair guide.

  1. Turn off electrical power to your furnace either at the furnace or at the electrical box
  2. Close the gas valve
  3. Remove the flame sensor mounting screw
  4. Gently pull out the flame sensor
  5. Visually inspect it
  6. If the sensor insulation is intact but looks burnt and sooty or has a small amount of corrosion on it, the sensor might simply need cleaning (See below)
  7. If the porcelain insulation is cracked or pieces are missing, or if cleaning doesn’t fix the issue, the sensor must be replaced (see below)
  8. After cleaning or replacing the flame sensor, remount it
  9. Turn the power on, open the gas valve and light the furnace

Details on replacing and cleaning a flame sensor are below.

How to Clean a Flame Sensor

  1. Shut off power and gas to the furnace
  2. Remove the sensor
  3. Clean away soot and corrosion using emery cloth
  4. Reinstall the flame sensor
  5. Restart the furnace

Dirty and lightly corroded flame sensors can be restored to working condition with cleaning.

Purpose: The purpose is to sand away the dirt and/or corrosion to restore a clean metal surface.

Product: The best product for cleaning a furnace flame sensor is emery cloth in a fine grit. The finer the grit the better. Fine-grit emery cloth has a higher number than coarse or medium. The reason to use fine grit is to avoid the grooves in the soft metal of the flame sensor that coarse emery cloth can cause. Grooves collect carbon from the burner flames. As carbon builds up, the sensor will fail again and require cleaning.

You want to leave a smooth, polished surface on the metal flame sensing rod.

Emery cloth is available at any hardware and from online sellers.

Process: Simply wrap the cloth around the sensor and work the cloth side to side and up and down to remove the debris. This video shows how to remove the sensor and clean it with emery cloth.

How to Replace a Flame Sensor

  1. Remove the mounting screw, and pull out the sensor
  2. Detach the wire from the sensor
  3. Push the new sensor into the opening
  4. Secure it with the mounting screw
  5. Attach the wire to the new sensor

How to know if the sensor is broken: If you remove the sensor and find that it is visibly damaged as shown in the image below or if cleaning it doesn’t fix the issue, it should be replaced.

Where to buy a furnace flame sensor: Local furnace parts suppliers should have the sensor you need. Take the part with you along with the furnace information. The brand, serial number and model number can be found inside the access panel.

Online sellers: If you don’t have a local supplier, there are many online sellers like Supply House and Repair Clinic. A call to the supplier is the quickest way to get the right part, or you might be able to find it by searching the site using the make and model of the furnace.

Universal flame sensors:  You might not need an exact replacement part. Universal sensors are available too. They are available at Home Depot, Menards or other building supply store.

Video tutorial: This video from a Canadian supplier shows how easy it is to locate, remove and replace a flame sensor.

What if Your Furnace Still Doesn’t Work?

This quick and easy repair works most of the time when your furnace has the most common symptom – starting and then shutting down within a few seconds.

If cleaning the sensor doesn’t work, replace the sensor. If the replacement doesn’t work, then the sensor likely isn’t the issue. Your best bet is calling a certified HVAC repair company to determine the cause of your furnace malfunction.

Free furnace repair estimates: We can assist you in getting free, no-obligation estimates from several of the top furnace repair companies in your area. It takes just a couple of minutes to provide the information needed, and you’ll soon have guaranteed repair estimates from pre-screened, licensed furnace repair experts to consider.

The service is free.

Cost of Furnace Flame Sensor Replacement

DIY flame sensor repair cost: If you clean the sensor, and that does the job, your price can be less than $5 for emery cloth.

If you replace it yourself, the sensor will cost $6 to $75 based on the furnace model and whether you buy an OEM part or a universal sensor. Most furnace flame sensors cost less than $40.

Pro furnace flame sensor repair cost: When you have to call a furnace repair company, you’ll pay a minimum service fee plus the cost of a sensor if it needs to be replaced.

  • Minimum service fee:$65 – $150 for cleaning or replacement
  • Flame sensor part cost, if needed:$6 to $75 in addition to the minimum service fee

If you only need the sensor cleaned, you’ll pay the minimum service fee. When it also has to be replaced, you’ll pay the minimum service fee plus the cost of the flame sensor.

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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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