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What Can Cause a Furnace to Stop Working?

What can cause a furnace to stop working? You might be surprised to learn it’s not always the furnace’s fault. A powered-off switch, faulty thermostat, or air duct issues could be to blame.

Let’s look at some of the most common culprits behind a malfunctioning furnace.

What can cause a furnace to stop working? 8 potential causes

Furnace switch powered off

Furnace power switches often look like light switches. It’s easy to turn them off, thinking you’re turning out the light. Before checking for other issues, make sure the power switch is turned on.

Furnace door safety switch

This is one of the first problems you should rule out before checking everything else. The safety switch stops the furnace from working when the door is removed.

If you’re recently removed the furnace door for whatever reason, make sure the safety switch has been reactivated.

Dead batteries in the thermostat

This is obviously a problem with the thermostat, not the furnace itself. Check to see if your thermostat is working. If there’s no power, change the batteries. Lithium batteries last much longer than alkaline batteries.

Tripped circuit breaker or blown fuse

If the thermostat is on but there’s no warm air coming through the vents at all, check your circuit breaker or fuse box. If the breaker has tripped, you’ll need to reset it. Or, change the fuse.

Broken thermostat

If you’ve changed the thermostat batteries and your furnace still isn’t working, your thermostat could be broken. Several issues could cause a thermostat to malfunction, including:

  • Faulty wiring
  • Dust and debris
  • Malfunctioning transformer
  • Broken heat anticipator

If your thermostat isn’t functioning properly, consult your local HVAC experts for recommendations on new equipment and installation services.

Clogged filters

This is one of the most common culprits behind a malfunctioning furnace. If your furnace isn’t producing enough heat, chances are the filters are clogged.

Furnace filters trap dirt, fine particles, and other debris. Warm air can’t flow through clogged filters. The furnace might not stop working outright, but it will be much less efficient.

Filters should be changed once every season, about every 3 months. Annual maintenance can prevent filters from getting clogged and save you money on repairs and lost energy.

Air duct issues

Inconsistent heat is a tell-tale sign that your air ducts are the issue, not the furnace. If you notice one room is significantly warmer or colder than the others, your air ducts are likely leaking.

Inspect your ductwork for any obvious signs of damage. If your furnace is working properly, you should be able to feel heat escaping through the leaking ducts.

Sealing the leaks with duct tape could be a temporary fix. But there’s always a chance that a small leak will be left behind. Your local furnace repair experts have the tools and equipment needed to permanently seal leaks.

If you can’t find any leaks, the problem could be as simple as opening the dampers that control airflow into each room. But if they’re open and the heating is still uneven, it’s a good idea to call your local HVAC service. Some leaks are small and hard for homeowners to detect.


Even the best-maintained furnaces will need replacing eventually. If your furnace is more than 15 years old and suddenly stops working, it’s time to replace it. You’ll end up paying more to repair an old furnace than to replace it.

Related: How long does a furnace last?

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