Furnace Won’t Turn On – Furnace Troubleshooting

If you’ve read some of our articles, then you know that All Systems Mechanical is a small, U.S. Veteran-Owned business located in Southern California, and we pride ourselves on giving straight, honest answers; we have built our reputation on it.  A malfunctioning furnace might not be a big deal in Santa Clarita, California, but when you start moving up north it can be downright dangerous when a furnace won’t turn on.  In fact, if you get far enough North and your furnace won’t turn on, you run the risk of all sorts of secondary problems such as 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 your furnace by yourself and avoid a pricy repair bill.  In this article we will address the step-by-step troubleshooting process that you should 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:

  • Heater Blowing Cold Air.
  • Furnace Keeps Turning On and Off Repeatedly

Troubleshooting: Your Furnace Won’t Turn On

In this section, we will go through a methodical, step-by-step process to help you troubleshoot your furnace.  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 check when you call them out to yourself.  These are also the most probably causes!  So, get out your work gloves and toolkit – all of these steps together shouldn’t take you more than about 20 minutes.

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

Even if you run 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 or out of your furnace’s 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: Nest vs Honeywell Lyric.  Here are the steps for checking the electrical input to our furnace.

  • Start by checking other electrical components throughout the house.  If they aren’t working either, then call your electric company.
  • 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”).
  • 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.  NEVER TAPE A CIRCUIT BREAKER ON.  You run the risk of starting an electrical fire!  Your breaker popped for a reason.  Time to call out your local HVAC contractor.

Step 2: Check Your Thermostat If Your Furnace Won’t Turn On.

The second step in troubleshooting when a 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 or their intelligence, but I’d say that the simplest answer seems to be the problem about 40% of the time.  Use the following steps:

  • Check that the thermostat has power to it.
  • Check that the thermostat is on.
  • 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 system turns on.  If it does, then set the desired temperature and your all done.  If your furnace still won’t turn on, then move on to the next step.

Step 3: Check the Condensate Pan of Your Furnace.

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 air filter is clogged because it has not been replaced as frequently as it should have, it could 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: Filter Change.

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 this 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 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 light can go out and will need to be relit.  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.  For more information on gas leaks, try: Furnace Smells Like Gas.

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: this will explain it to you: Furnace Won’t Turn On – Check Your Pilot Light

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 a flame in it.  If you remember from Step 5, your furnace will not 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 in your furnace’s operation manual, calling the manufacturer or by running a Google search.  Then, here is a brief video on how to clean your furnace’s flame sensor.


Final Thoughts On When Your Furnace Won’t Turn On

At the end of the day, when your furnace won’t turn on it can be frustrating.  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 here and your furnace still won’t turn on, it’s time to call in your local air conditioning repair company.  Should your system need to be replaced, here is a useful article: The Top 10 Furnace Brands.  At All Systems Mechanical, we service Santa Clarita and Greater Los Angeles, including Westlake Village and Thousand Oaks.  But in case you don’t live near us, here is a good article on how to choose a reputable HVAC contractor.  If you live in Santa Clarita, San Diego or other cities in Southern California

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