9 Reasons Why Your Furnace Won’t Stay On

Gas furnace won’t stay lit?

Some reasons why your furnace burner won’t stay are on:

  • Pressure switch
  • Circuit board is faulty
  • Dirty Flame Sensor
  • Airflow is restricted
  • Not enough combustible air
  • Gas it out
  • Heat exchanger is overheating
  • Malfunctioning thermostat
  • Thermocouple is bad

Below you’ll find out how to identify each issue and how to fix it.

Let’s get started.

9 Reasons Why Your Furnace Burner Won’t Stay Lit

1. Pressure switch

Your pressure switch is a round object that’s bolted on the outside of your furnace. It’s located close to the draft inducer motor.

If the pressure switch becomes blocked by debris or insects, it can cause your furnace to stop functioning.

Another reason your furnace won’t stay lit is that the pressure switch is on its way out or faulty.

2. Circuit board is faulty

Not all furnaces have circuit boards, but if yours does, it’s possible that your circuit board has a short.

It’s best for a licensed HVAC technician to inspect your circuit board as you’ll need specialized tools to troubleshoot your furnace’s electronics.

3. Dirty Flame Sensor

The flame sensor on the furnace checks if there is a fire, and if there isn’t, it turns off your furnace. This is to prevent any explosions in the combustion chamber.

You can remove your flame sensor (it’s in front of the pilot light) and give it a cleaning with light grit sandpaper.

Be careful as to not grind down the flame sensor, and only use the sandpaper to remove any caked-on dirt and grime.

If your furnace is a direct ignition furnace, then you will not have a pilot light.

If your flame sensor is clean, then your pilot light may be faulty.

The other possibility is that you have a faulty flame sensor.

4. Air Flow is Restricted

Your furnace is designed to turn off once the burners reach a certain temperature.

This is so the furnace can cool down in between each cycle and prevent overheating. When your furnace cools down, your burners should ignite again.

If your furnace’s airflow is restricted, it will not be able to continue this cycle properly.

Either replace or wash your filter (if it’s reusable) and see if that fixes the issue.

A faulty blower motor can also be a reason why your furnace is having issues with airflow.

5. Not Enough Combustible Air

If the room your furnace is in is fairly small, this can prevent your furnace from being able to stay lit due to lack of oxygen.

You can try to add some venting to the room or even try opening the door or window to allow some fresh air to come inside.

6. Gas is Out

If the gas supply to your home is running low, then your furnace will not be given enough gas flow to stay lit.

If you have a tank of gas, you can run some water down the side and then run your hand down the side.

Once it feels cool to the touch, that’s the amount of gas you have left.

If it’s near the bottom, you’re nearly out of gas and you’ll need a refill.

7. Heat Exchanger is Overheating

Your furnace has a limit switch that shuts down the furnace if the heat exchanger gets too hot.

This can be caused by a dirty filter but is also a symptom of an overheating heat exchanger.

Be careful as a cracked heat exchanger can lead to carbon monoxide leaks.

8. Malfunctioning thermostat

If your thermostat is not calibrated properly, then your furnace may think that the desired temperature has been hit.

Try replacing the batteries on your thermostat, if that doesn’t work, then you may need to purchase a new one.

9. Thermocouple is bad

The thermocouple is what senses if there is heat coming from the pilot light.

If it’s broken, then it will tell your furnace to close off the gas valve. Check if the pilot light is on, and if it won’t stay lit, then you’ll need to replace the thermocouple.

When to Call An HVAC Technician?

If you’ve tried the above 9 fixes for your furnace and it still won’t stay lit, you should call an HVAC technician to come to take a look at your furnace.

You’ll be able to rely on our experienced technicians that are ready to help get your furnace in tip-top shape.

Furnace turns on and off every few minutes? That’s an entirely different problem that needs to be diagnosed.

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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