3 Reasons Why Your Furnace Isn’t Firing Up

May 16, 2025

That whooshing sound of your furnace starting up, and the contented hum that follows as your house gets warmer… you might not realize just how comforting these sounds are until your furnace suddenly isn’t making them. What’s gone wrong? Why isn’t your furnace firing up? There are many possible reasons, but they all fall within three basic categories. Let us help you figure it out.

Something You Can Fix Yourself

The best outcome here would be if you can get your furnace up and running without any help from a professional. We never encourage people to attempt DIY repairs, but this is a simple job you should be doing regularly anyway. Clean your furnace’s air filter! A clogged air filter can restrict airflow so much that the furnace won’t work at all, or starts up but immediately shuts off because it’s overheating.

Something Other Than Your Furnace

When the furnace won’t heat your home, the obvious assumption would be that something is wrong with your furnace. But what if that’s not the case at all? There are two other things that could be preventing the furnace from getting going.

  • The Gas Line: This is most likely to be the issue if your furnace won’t start the first time you turn it on in the fall, but can happen at other times as well. A stuck valve, a blockage, a kink, a leak… anything that would prevent the gas from flowing properly will stop the furnace from starting. If your furnace won’t ignite in San Antonio, TX, and you smell gas or suspect a gas line problem, get help right away!
  • The Thermostat: Your furnace could be perfectly functional, but if it doesn’t get the signal to come on, your house will stay cold. Check that you haven’t accidentally set your thermostat to “off” or “cool” or unintentionally activated a vacation setting that lowers the temp. These things happen! You can also try replacing the batteries. As they lose charge, they can sometimes send inconsistent signals.

Dirty or Failing Furnace Components

Now we come to the actual furnace. But don’t worry, it might be that all your components are still doing okay, just dirty! Combustion is a dirty business, and if certain components don’t get disassembled and thoroughly cleaned during annual professional maintenance, they can prevent your furnace from properly firing up. In some cases, though, components do need to be replaced.

  • The Pilot or Igniter: Dirty buildup here can stop a flame from starting or from staying lit.
  • The Flame Sensor: This component won’t allow gas to be released to the burners unless it senses a flame. If it’s too dirty, the flame could be right there, and the flame sensor would still shut down the furnace because it couldn’t detect it!
  • The Burners: Carbon from combustion or other particles caked onto the burners can stop them from drawing in oxygen. No oxygen, no fire!

Avoiding Future Furnace Failure

The best way to cut down on furnace issues and the need for repairs is to have professional maintenance done every single year. All these components will be cleaned, moving parts will be lubricated, and everything that concerns your safety—like that gas line—will be thoroughly inspected and tested. If you haven’t had maintenance done this year, schedule it now!

Whether you need maintenance or repair, or just have a simple question about how to change your furnace’s air filter, we’re always happy to hear from you.

Our professionals are ready to take care of you when you reach out. Contact Cowboys Air Conditioning & Heating today to schedule an appointment with our professionals.

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