How To Bypass Flame Sensor On Furnace

The flame sensor is a critical component in modern gas furnaces and boilers. It keeps the heating system on based on whether there’s a flame on the burner, effectively protecting us from gas poisoning and fires. 

Without a mechanism to consistently verify the presence of a fire on the burner, the gas getting to the burner may accumulate within the gas and even spread into the house, creating a massive fire risk. 

Unfortunately, the flame sensor can malfunction without warning. When this happens, you may need to clean or replace the sensor. 

However, sometimes the sensor may refuse to work (no matter how hard you try). In such circumstances, a few people might be tempted to bypass the sensor to keep the furnace running until they can replace the sensor. 

So, can you bypass the flame sensor? If so, how? And, is it safe? But, more importantly, what is the alternative solutions? 

What’s the Flame Sensor

The flame sensor is a small safety device located within your furnace’s burner assembly. It’s typically a thin, usually bent (though some are straight) metallic rod and sits in front of the flame stream inside the furnace. 

What Does the Flame Sensor Do?

The primary purpose of the flame sensor is to confirm the presence of a flame on the burner. The feedback helps the furnace determine whether or not to keep pumping gas to the burner system.

If the flame sensor returns a “flame present” feedback, the furnace can confidently continue pumping gas to the burner(s), knowing that the gas is being used up in the combustion process.

However, if it returns a “no flame” feedback, the furnace control system/board must instantly cut off the gas supply to prevent gas accumulation within the furnace and the home. 

This is important as accumulated gas can poison the people in the house. It also creates a massive risk of fire and explosion. 

Signs of a Bad Flame Sensor

Unfortunately, the flame sensor can malfunction or even stop working altogether. This can be due to dirt buildup or a defective sensor. 

The signs of a bad flame sensor aren’t obvious because the same signs may also point to a different furnace issue. However, you can be pretty confident that your furnace issues originate from a bad flame sensor if you observe the following;

Furnace lights then shut off almost immediately 

When you light the furnace, and it shuts off within about 10 seconds, you almost certainly have a bad flame sensor. That’s because gas furnaces only wait about 10 seconds for the flame sensor to detect the flame.

After ten seconds, the sensor will return a “no flame found” feedback if a flame isn’t found. Thus, the control board will cut off the gas supply and consequently shut off the furnace. 

The porcelain on the flame sensor is cracked

The standard flame sensor has a porcelain base that serves as insulation. If you notice cracks on the base, it’s another tell-tale sign that you might have a bad flame sensor.

Usually, the porcelain base will crack due to electrical issues within the sensor. So, if you find crack lines on the base, it’s likely short-circuited. 

The flame sensor is sooty or corroded

The flame sensor relies on its sensitivity to fire to generate the voltage potential to detect the flame on your burner. Thus, if your sensor is sooty or corroded, it may lose that sensitivity, causing it to return erroneous and misleading feedback. 

This is how you fix yellow flame on gas furnace

Bypassing the Flame Sensor

We strongly advise against bypassing the flame sensor as it exposes you to massive risk. Remember that the flame sensor is present on the furnace to protect you from poisonous gas (propane or natural gas). 

Nevertheless, if you must bypass it (momentarily), perhaps to confirm that the furnace works when you remove the sensor out of the equation, then you need a modified “bypass circuit.”

A modified flame sensor bypass circuit comprises two alligator clips, two 8-inch pieces of insulated copper wire, a 1 -¼ watt 499-volt diode, and 1-1/4-watt 100k Ohm resistor. These components can be soldered together in sequence. 

Once you have the bypass circuit ready;

  1. Unplug the appliance first: This is very important. You must never bypass the furnace with the power ON. Alternatively, turn it off at the breaker and have someone close by to help you confirm that the breaker is OFF.
  2. Turn off gas supply: This is also very important. Otherwise, you expose yourself and other home occupants to dangerous gases and your home to the risk of fire. Make sure the gas valve is perpendicular (90 degrees) to the gas line before you start.
  3. Replace the sensor with the bypass circuit: This is a delicate process. However, generally, you need to disconnect the O and N pins from the PCB board and replace them with the bypass switch. 
  4. Run the furnace: Now, restore electric power and turn back the gas supply. Then switch the furnace back ON. It should work as if the flame sensor is dipped into a healthy flame and functioning normally. 

If you can pull it off, the bypass switch can be a useful way to prove beyond doubt that your flame sensor is compromised. 

However, we strongly advise against using a flame sensor bypass switch as it’s too dangerous and unnecessarily risky. 

It’s Safer to Wait and Replace than Bypass

If the flame sensor refuses to work even after cleaning, consider the multimeter to test to see if it conducts electricity. If it doesn’t, it’s best to wait until you can replace it, even if you have to spend a night in the cold. Not only is it safer, but it also costs less in the long run. 

The best part is that replacing a flame sensor switch is a straightforward process if you purchase the correct replacement sensor. 

Just remove the old, faulty sensor as we explained earlier, fix the new one in its place, and set/screw it in place. Then turn ON power and gas and switch ON the furnace to begin enjoying the warmth once again. 

How to Clean the Flame Sensor

As we’ve seen, sometimes the flame sensor can fail simply because it’s covered in soot or too corroded. Or it could be covered in a thin layer of dust and debris. Cleaning it with a ball of soft steel wool can restore its function. 

So, before you think about replacement, consider cleaning it to see if the problem goes away. Here’s how to proceed;

  1. Shut off the furnace: Shut off the electric power at the breaker and turn off the gas supply at the gas valve. The valve is OFF when it’s perpendicular to the gas line. 
  2. Remove the burner assembly door: The exact steps here will depend on the type of furnace. However, in most cases, you need to unscrew then lift the door off. 
  3. Remove the flame sensor: The flame sensor is often mounted with a single ¼ -inch hex head screw. Remove this screw and slide the sensor out gently.
  4. Clean it thoroughly: Rub the metal part (and no other part) with a very soft grit pad. Be very gentle, keeping in mind that excessive force can damage the wire. Once you’re done, use a soft dry cloth to wipe out the excess dirt. 
  5. See if it works: Put the sensor back in place, ensuring to screw it on tightly, and return the burner door. Then turn on the gas supply (it’s ON when the gas valve lever is parallel to the gas line) and switch back power at the breaker. Once done, switch ON the furnace and give it a full minute to see if it works. 

Does it work? If it works, then the sensor was merely dirty or too sooty to sense the flame accurately. You can now stop and enjoy the warmth. However, if it doesn’t, you likely have a defective flame sensor. You need to test it to verify that it’s bad. 

How to Test Your Flame Sensor

Testing the flame sensor will tell you whether it conducts electricity. You need a multimeter and screwdriver for this process. Once you have both; 

  1. Remove the flame sensor safely: Follow steps #1 to #3 above to safely remove the flame sensor from the burner assembly, not forgetting to switch off the power and turn off the gas supply before you begin. 
  2. Determine the resistance without fire: Touch the multimeter probes to the sensor’s white and blue wire ports. It should show a very low resistance. 
  3. Check the resistance with fire: now, take the sensor and press it against a 60-watt (or higher) bulb for at least 10 seconds. Then check the resistance reading again. This time you should get a much higher reading. 

If the multimeter readings are the same, i.e., the resistance is the same whether the sensor is cold or heated, it is defective. 

Summary

Yes, you can temporarily bypass the flame sensor switch with a bypass circuit. However, it’s too risky and unnecessarily dangerous. So, why not just replace the flame sensor instead of putting yourself and your loved ones in danger? It costs less than $40 and is a straightforward DIY job.