Furnace Won't Turn On - 9 Troubleshooting Tips for a Broken Furnace

Furnace Won't Turn On - 9 Troubleshooting Tips for a Broken Furnace

If your furnace won't turn on, it can be frustrating.  If you’ve read some of our articles, then you know that ASM-air started as a small, U.S. Veteran-Owned business that built its reputation on giving straight, honest answers; this article will be no different.  A malfunctioning furnace might not be a big deal in California, but when you start moving up north it can be downright dangerous when your furnace won’t turn on.  In fact, if you get far enough north, you run the risk of all sorts of secondary problems, like frozen water lines and condensation problems (not to mention it’s just plain cold)!  But don’t call your local furnace repairman just yet, because there are a couple of things that you can do to troubleshoot a furnace by yourself, and hopefully avoid a pricy repair bill.  In this article we will address the step-by-step troubleshooting process that you can use when your furnace won’t turn on, starting with the basics, and working our way up to more advanced furnace troubleshooting techniques.

Note: This article addresses when a furnace won’t turn on.  If your furnace is turning on, but blowing cold air, or keeps turning on and off, try these articles instead:



Furnace Won’t Turn On: Troubleshooting

In this section, we will go through a methodical, step-by-step process to help you troubleshoot when a furnace won't turn on.  Although I can’t guarantee that this will fix your problem, I can tell you that these are the first steps that any qualified HVAC technician will take when you call them out to your home.  These are also the most probable causes!  So, get out your work gloves and toolkit – all of these steps together shouldn’t take you more than about 30 minutes.


Furnace won't turn on troubleshooting flow chart

Step 1: If Your Furnace Won’t Turn On, Check the Circuit Breakers

If your furnace won't turn on, the first step should always be to check the circuit breakers.  Even if you own a furnace that has propane or natural gas as a source of fuel, some of the components require electricity to operate!  

Think about it; electrical ignitors and various inputs into your furnace thermostat all need power to run.  The more advanced your system is, then the more susceptible your system is to power outages.  Some of the newer smart thermostats are especially susceptible.  

For more on the newest generation of thermostats, see:

If your furnace won't turn on, here are the steps for checking the electrical input to your furnace.

  1. Start by checking other electrical components throughout the house.  If they aren’t working either, then call your electric company.
  2. Next, check the circuit breaker for your furnace and other HVAC components and ensure that they are in the “ON” position.  These will be located on your main power distribution panel (aka your “circuit breaker panel”).
  3. Next, check for a circuit breaker on your furnace itself, or a light switch located right by your furnace – some units have redundant circuit breakers on your furnace itself, or a light switch that controls the power to your unit.

If any of these circuit breakers are popped, reset them once (and only once!).  Never reset a circuit breaker more than once – it is tripping for a reason, continuing to reset it not only won’t fix the problem, but runs the risk of damaging your system or starting a fire.  


You run the risk of starting an electrical fire!  Your breaker popped for a reason.  If your circuit breaker keeps tripping and your furnace won't turn on, then it's time to call out your local HVAC contractor.

You might also be interested in:

furnace won't turn on - goodman furnaceStep 2: Check Your Thermostat If Your Furnace Won’t Turn On

The second step in troubleshooting when your furnace won’t turn on is to check the thermostat itself.  Some thermostats will lose their programming and reset if they loose power, even for a second.  If this happens, they might return to their default settings.  I honestly am not trying to offend anyone's intelligence, but I’d say that the simplest answer seems to be the problem about 40% of the time.  Use the following steps:

  1. Check that the thermostat has power to it if your furnace won't turn on.
  2. Check that the thermostat is on.
  3. Check that the thermostat is in the “Heating” mode of operation.

Set the temperature you would like – for test purposes, just set about 10 degrees hotter than the current temperature to see if the furnace turns on.  If it does, then set the desired temperature and you're all done.  If your furnace still won’t turn on, then move on to the next step.

Step 3: Furnace Still Won't Turn On?  Check the Condensate Pan of Your Furnace

If your furnace won't turn on, next is to check your condensate pan.  Condensate pans (aka drain pans) accumulate water that has been removed from the air by your air conditioner or furnace.  Although drainage pans are associated more with air conditioners, furnaces still produce condensation (which includes acid in it), so they will likely have a float switch and a condensation pump as well.  

If it is operating properly, the condensation should drain out or be pumped out via a pump located in the pan or a collection reservoir (about a half gallon in size).  If your condensate pan has standing water in it, follow these steps:

  • If your pan has a drain (just look at it, you’ll see PVC coming out of it that is gravity fed), check that the drain is clear.  You can buy special tablets at your local department store which will keep these drains clear.  Then drain the fluid and check for normal furnace operation.
  • If your furnace’s pan has a pump (you will see a mechanical pump at the bottom of a water line that comes out of your pan), or it will have a collection reservoir with a tube feeding into it, then there will be a float switch.  Check to see that the float switch is in the “down” position.  If it is stuck “up,” clean it and check for proper movement (take care to wash your hands thoroughly after coming in contact with collected condensation, as it contains acid).  If it is stuck up with water in your pan, then you will likely need a new pump.  Pushing down on your float and holding it can verify if this is the problem.  If this is the problem, your furnace will turn on within about 30 seconds to a minute.

Your furnace will not run with a triggered float switch, so if the float switch is up, this is the likely cause and your furnace won’t turn on.

Step 4: Check Your Air Filter if Your Furnace Won't Turn On

Your furnace will not turn on if your air filter is clogged.  If it is clogged, it has not been replaced as frequently as it should have!  This can cause your furnace to turn off as a safety feature (to prevent overheating).  Your next step should be to check your air filter.  If it is extremely dirty, then replace it immediately.  For more information on this, try:

Step 5: Check Your Blower Motor if Your Furnace Still Won’t Turn On

You don’t need to know what a blower motor is to check that it is working properly.  If your furnace won’t turn on, go to your furnace (if it is accessible to you), and peak in the little plastic window on the side of it (most modern furnaces will have a window).  You are looking for a blinking green light.  

If the light is doing anything other than blinking green, then you may have to call out a professional.  If it is blinking green, then that’s usually good; move on to the next step.

Step 6: If Your Furnace Won’t Turn On, Check Your Pilot Light

If your furnace does not have a pilot light, move on to the next step.  However, even many modern furnaces still run pilot lights.  If gas or propane flow to this pilot light is interrupted, even for a moment, the pilot light can go out and will need to be relit.  This is why your furnace won't turn on.  

Many modern furnaces will not turn on if your pilot light is out to avoid pumping massive amounts of gas or propane into your house...this, uh, wouldn't be a good thing.  For more information on gas leaks, try:

If your pilot light is out, or if you don’t know how to check your pilot light, avoid a costly call to a furnace repair company and watch a short video instead: this will explain everything to you:

Step 7: If Your Furnace Won’t Turn On, Check Your Gas or Propane Supply

If you want to continue troubleshooting and your furnace still won’t turn on, then it’s time to check your gas or propane.  The easiest way to do this is to check another gas appliance in your house, such as your stove.  If it is not working either, then you’ve found your problem and it’s time to call the gas company or get the propane tank filled.

Step 8: Furnace Won’t Turn On – Check Your Ignition Sensor

Now you are starting to get into the weeds a bit.  Unless you are a veteran do-it-yourself’er, I’d probably call in the pros now and leave this to your local HVAC contractor.  But if you are interested, your ignition sensor (aka flame sensor) is a safety feature of your furnace that actually tells your furnace whether or not there is actually a flame inside of it.  

If you remember from Step 5, your furnace won't turn on if it does not sense an ignition inside of your heat exchanger (what is a furnace heat exchanger?).  If this sensor gets dirty, however, then it will not function properly and needs to be cleaned.  

Locate your furnace’s flame sensor by looking inside your furnace operating manual, calling the manufacturer, or by my favorite...running a Google search.  Then, here is a brief video on how to clean your furnace’s flame sensor.

Step 9: Don't Overpay for Your Your New Furnace  

Hopefully you were able to get your furnace up and running again, but if you weren’t then unfortunately, it might just be time.  However, I would definitely consult an HVAC technician before doing so.  Here’s an online installation price calculator that will help you get the best price:


Review of Troubleshooting Steps for When A Furnace Won't Turn On

Ok, so by this point you have figured out how to get your furnace back on...or you've called a reputable professional.  Either way, good luck on your journey.  For those of you who are "skimmers" when you read, here's a review of the troubleshooting tips we went through to troubleshoot when a furnace won't turn on:

  1. Check circuit breakersreakers.
  2. Check your thermostat.
  3. Check the condensate pan (the pan that catches water under furnace).
  4. Check the air filter.
  5. Check the blower motor.
  6. Check your pilot light.
  7. Check your gas or propane supply.
  8. Check your furnace ignition sensor.
  9. Call a professional.

Final Thoughts On When Your Furnace Won’t Turn On

At the end of the day, it can be frustrating when your furnace won’t turn on.  But if you follow the troubleshooting steps above, you will be able to find the problem about 80% of the time.  If you’ve gotten to this point, and still...your furnace won’t turn on...then it’s time to call in your local AC repair company.  Should your system need to be replaced, here is another useful article: The Top 10 Furnace Brands.  For all other questions, try our online calculator; click below for more information:



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Timothy Kautz
About the author
Tim K.

The University of Virginia - 2005 / The Wharton School of Finance - 2016 / U.S. Naval Aviator 2005-2015. At All Systems Mechanical air conditioning and heating, we believe that the experience our clients have is every bit as important as the products they receive. Simply put, our results speak for themselves, and we'd be happy to help. If you're in the market for a new AC or furnace, make sure that you get a fair price! Try our online calculator; click the tab on the top of this page for more information.

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