A furnace flame sensor does what it says literally- to sense flame. It makes sure there is fire once the furnace will start. In the absence of fire, the flame sensor sends a signal to shut off the gas or fuel valve to prevent dangerous gas build-up that can cause an explosion.
Sometimes, your furnace flame sensor can go defective and damaged. If you’re in doubt if it is your flame sensor that is acting up and causes troubles to your unit, check out these common signs of a faulty flame sensor:
- Furnace turns on then off after a few seconds
- Flame Sensor appears to be corroded or dirty
- The porcelain on the sensor is cracked
Either of the three signs will cause your furnace to operate in short cycles- meaning, it will turn on then off in a few seconds. This happens because a dirty or damaged flame sensor won’t be able to sense the presence of heat and will command the furnace to shut down. This can leave your home without heat in a cold season which can be very annoying.
Before we go through the basic troubleshooting steps, let’s first locate the flame sensor inside your furnace.
Furnace Flame Sensor Location
Remove the furnace cover. Remove the screws or knobs that hold the furnace cover in place.
Look outside the burner assembly. The flame sensor is mounted just outside the burner assembly with wire/s attachment. The sensor sticks up through the housing and into the area where the flame is burning.
Look for the flame sensor. It is a thin metal rod that may be straight or bent around the tip depending on the sensor type or brand. It is mounted by porcelain insulation on the other end.
Furnace Flame Sensor Step-by-step Repair and Replacement Guide
Shut off electrical power to your furnace
You can switch it off through the toggle switch on/near your furnace or through its dedicated circuit breaker box, in the absence of a switch.
* NOTE: Make sure to shut off MAIN POWER, and not the thermostat alone.
Close the gas valve
If your gas valve is not electrically controlled, shut off the gas unit as well.
Locate the flame sensor
Locate the flame sensor at the back of the furnace and in the path of the burner.
Remove the hex screws
The flame sensors are typically kept in place by one or two ¼” hex head screws.
Slide the flame sensor out
Gently pull the sensors out from the burner bracket or assembly.
Detach the connected wires
For some units, you may have to detach the wires that are connecting to the sensor so you can clean it well. This is not necessary but detaching the wires will give you ample space and flexibility in cleaning the sensor later on.
Visually check through the flame sensor
1. If soot, corrosion and carbon build-up is visible, your furnace flame sensor just needs cleaning. This happens very commonly. Good news is, cleaning a furnace flame sensor is very easy and it can be done anytime with tools that are easily available at your home.
We have a complete guide on How To Clean A Furnace Flame Sensor on a separate blog to help you out in your dirty flame sensor problem.
2. If there is/are visible crack/s on the porcelain insulation or an obvious damage, the sensor must be replaced.
Replacing your flame sensor is not a difficult task, simply redo the above steps– shut furnace and gas valve off, remove the flame sensor from the assembly and detach the wires. Skipping the cleaning part, proceed by mounting the new furnace flame sensor by doing the above steps now in reverse.
Check if the problem persists.
Once a clean/ new furnace flame sensor is remounted, check if the furnace now works properly. Turn the power and the gas valve on and see if the furnace stays on for at least one power cycle.
Where To Buy Furnace Flame Sensors
In replacing your flame sensor, you have to make sure to purchase the correct part replacement.
Local HVAC Shops. Various local HVAC providers mostly have flame sensors available. In Canada and North America, ACEHVACPARTS offers a broad list of furnace replacement flame sensor that you can choose from.
If you’re not sure which part to get, check your unit’s information inside the access panel, and contact us with your brand, model and serial number so we can suggest the best match for your unit.
Local Supply Store. Should you choose to use a universal flame sensor as a replacement, they are widely available on Home Depot, Menards or other building supply store near your area.
How Much is the Cost of Furnace Flame Sensor Repair and Replacement
For Do-It-Yourself repair and replacement. If a dirty flame sensor is the only issue, all you need is light grit sandpaper, steel wool, or emery cloth which will cost you less than $5.
For replacement, buying a new flame sensor will cost you less than $10 to $100 depending on your unit model and if you’ll get an OEM or a universal part. However, most flame sensors cost $30 – $50 in the market.
For professional repair and replacement. If this is the case you have to pay the minimum service fees of the furnace repair company that goes around $65 – $150, plus the cost of your new flame sensor if its needs replacement which is less that $10 to $100.
Thus, it is very much economical to do your furnace flame sensor repair and replacement yourself. Besides, we have already walked you through the steps and it is not a difficult job at all.
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