Over the years gas furnaces have proved to be some of the most reliable means of home heating. That, however, does not mean they are bulletproof and will always come on and work when you need them. Like any type of mechanical appliance, they are prone to occasional breakdowns. One of the most common problems on forced air gas furnaces is their failure to ignite and come on. It can be a very frustrating experience that literally leaves you out in the cold.
Of course, when your gas furnace fails to ignite you can pick up the phone and call your HVAC repair person. That service call is not going to be free for sure; this is especially true if it’s after normal business hours. The alternative is to save yourself some money by trying to repair your furnace ignition problem yourself.
This guide is mainly talking about furnace not igniting problems. Read our How to Troubleshoot Top 12 Furnace Problems for more info.
Furnace Ignition Failure Easy Troubleshooting
There are some very simple troubleshooting steps, that anyone with even a light mechanical background can do themselves. If you are successful with one of these and get your furnace to come on, you may just save yourself quite a bit of money on an otherwise costly HVAC service call.
OK, three quick things that might be the problem if you’re new to the house or the HVAC system was worked on immediately before it won’t work. Make sure of three things:
1. The power switch on the furnace is in the ON position. It should look just like a light switch and be located on or right next to the furnace.
2. The gas valve in the gas line is in the ON position. The handle is long – and it should run parallel to the line when open/on. If it is perpendicular to the line, the gas has been shut off. Turn it on, and hopefully the furnace will fire!
3. The thermostat is turned to “Heat”
1. First Things First – Check Your Gas Supply
Troubleshooting a Gas Furnace When It Fails to Ignite One of the first steps in determining why your furnace is not igniting is to make sure it’s getting gas to it.
- Natural Gas Users
If you are on a city gas line you will not be able to do this step because trying to tell if you are getting natural gas to your heater is dangerous and should only be done by a professional HVAC technician. If you smell gas near your furnace when you turn it on there is a good chance that your gas supply is not the problem
- Propane or LP Gas Users
It’s much easier to determine if your furnace is getting gas if you are a liquid propane user. First of all, you need to go out into your yard and locate where your gas storage tank is. In the top center of it, there will be a domed area where its fill valve is. There will also be a gas pressure gauge.
Most propane systems will continue to supply gas until they are just about empty. However, some will start to sputter and possibly not supply enough gas for your furnace to fire when they get close to being empty.
If this gas pressure gauge reads 10% or above, then you should be OK; if not, call your gas company for a fill-up, and you have probably solved the reason why your furnace did not start up. This is an important step for propane gas users because if you call an HVAC technician before doing this they will charge you for a service call. This is true even if being out of LP gas was the only problem with your furnace.
How much is a service call? Expect to pay between $70 and $125 just to have a technician show up to say, “your propane tank is empty – and that’s your problem.”
Did you know? If your propane tank runs empty, your propane provider will probably want to do a pressure test of the system to check for leaks after filling the tank. You might be charged for the service – another reason to make sure you don’t run out of propane!
2. Check Your Air Filters
You would be surprised how many gas furnace users do not do this step when their heater does not start up and end up getting charged for an expensive HVAC service call. A gas furnace burns a mixture of air and natural or LP gas. If the air in your system is not flowing it cannot produce the required gas/air mixture and your home heating system will not ignite because of it.
Often times the reason air does not get to your furnace’s igniter is because it is restricted. The main reason for this air restriction is usually traced back to dirty air filters. So check them if your gas furnace fails to ignite when you turn the system on. It’s also a good idea to replace your furnace’s air filters at least once a year.
By the way, we should ask you if you’ve recently replaced your standard air filter with a high-MERV filter, like a MERV 12 or higher? That could be the problem, since MERV 12-MERV 16 filters significantly restrict air and might prevent furnace ignition.
We’ve created quite a lot more content on the issue of airflow and air filters. These resources might help you in troubleshooting a furnace that won’t start / won’t ignite.
- Where is My Air Filter Located?
- How to Tell If I Should Change Furnace Filter?
- Causes and Cures for Low Airflow
3. Ignition Source
There are two ways in which a furnace ignites the gas air mixture that comes into it. The oldest way is by a pilot light and over the last 30 years or so these have been replaced by electronic ignition systems. If these fail to do their job your furnace will not come on.
- Pilot Light
This is a small flame which is constantly burning inside your furnace. It’s so small it does not consume much gas. If you look at the bottom of your furnace you will see a tray where the burners are. Your pilot light is located to one side of this tray just slightly above it usually.
Look under your furnace to see if it’s lit. If not, you can use a long match to try and carefully light it again. If that does not work then you have a bigger problem.
- Electronic Furnace Ignition
With an electronic furnace igniter switch, your furnace is safer and does not use the small amount of gas that a furnace with a pilot light does. These igniters are located in about the same position as where we described the pilot light as being. They are made of a very lightweight metal alloy that gets super-hot when an electric current goes through it and this ignites the gas/air mixture coming into your furnace.
Look under your furnace and see if your igniter glows when your furnace tries to start up. You will also hear a clicking noise that is a sign your heater system is trying to come on. Repeated clicking in small intervals usually means your igniter is not coming on and working.
The only simply remedy for this that anyone can do is to very gently reach in and clean the igniter and the area around it. We can’t emphasize ‘gently’ enough because the igniter tip is very fragile. This will sometimes get rid of carbon that has built up around some sensitive ignition points and is keeping the system from coming on.
Turn off the furnace before trying to do this step!
See the FAQ Section below for igniter cleaning step-by-step directions.
If you turn your heater on and then move the thermostat to a higher setting several things should happen that will make the furnace ignite. The first sounds you will hear are that of the system’s fan which brings air into it to start the burning process. After that, you will hear the swoosh sound of the gas igniting. After a few seconds, hot air will then flow out of the vents in the rooms of your home.
None of this will happen of course if your thermostat is not working properly. What’s the easiest way to tell you to have a defective thermostat? None of the sounds we just described will take place because your broken thermostat did not communicate with your furnace that it’s time to come on. All will be silent near your gas furnace unit.
Fortunately, you know where to locate your thermostat. Here are the steps we suggest to effectively remove it and install a new thermostat:
1. Buy a new thermostat that will work with your system. It must have the functions you require such as two-stage heat or cooling, etc. Buying a compatible thermostat is discussed in detail in our thermostat buying guide.
2. Follow exact instructions for thermostat replacement. We strongly suggest you take a picture of the wiring connections, so that the new thermostat can be wired exactly the same – heating wire to heating connection, AC to AC, etc.
Bookmark the page, and when the time comes, click to our In-depth Thermostat Wiring Guide for Homeowners for 3 Simple Methods to Wire a Thermostat.
Pick HVAC has reviewed the top thermostats available including nest, Lux Geo, ecobee, Honeywell lyric, Carrier and more. You’ll find links to the right on the thermostat wiring page and the Buying Guide.
5. Draft Issues
Your furnace has a motor called a draft motor or draft inducer motor or blower. It creates the humming noise you hear when your thermostat first calls for heat and before you hear the “whoosh” of the burners firing.
Its purpose is to pull in air and blow it out the vent/chimney. It does this to ensure that the vent isn’t blocked, since if it were, poisonous carbon monoxide could back up into your furnace and leak into your living space.
Problem: If the draft motor is “dead,” the furnace won’t get the “all clear” signal, and it won’t light.
Solution: If the draft motor doesn’t come on, be suspicious that it might be the cause of your trouble.
Problem: In the event the vent is blocked by a bird or racoon nest or heavy snow/ice – we’ve seen all these and more – the furnace will get a “blocked” signal and not allow the igniter to fire.
Solution: If the draft blower starts – you hear it hum – but the furnace doesn’t ignite, the problem could be caused by a blockage in the vent. This is not super-common, but something to consider if other solutions don’t work.
6. The High Limit Switch is Open
The high limit switch, also called the limit switch, shuts down the furnace if it gets too hot, for example if the air filter is so clogged hot air builds up inside the unit because air can’t be circulated.
Check the filter, of course, and replace a dirty filter.
The next best step might be to contact an HVAC company using the 800 number on this page to have the situation evaluated. However, if you enjoy DIY, find the high limit switch. Many look like a small, round disc with two wires attached on the inside of the furnace.
Next, unfasten the switch, and gently pull it out. Look for the pin between the wires. If it is raised, the switch is tripped or open, and current to run the furnace can’t get through it.
Push the pin to close the switch, and turn the power to the furnace off for a few minutes to reset the error code. Then power the furnace again, and see if the furnace comes on. If it does, you might not have an issue again, but if the limit switch keeps opening, and the furnace won’t operate as a result, replace the switch.
If it keeps happening, then there’s a more significant problem with your furnace overheating, and that should be addressed before your heat exchanger cracks, and then you’ve got a pricey problem to deal with.
Note: Other limit switches are square and have no pin, but to “work,” they must have continuity. If you know how to check for that with a multimeter, you can troubleshoot this little device.
When it’s Time to Call an HVAC Technician?
Once you have done all of the above steps and your furnace still is not igniting you don’t want to try any other furnace troubleshooting unless you really know what you are doing with furnace heating systems. At this point, it’s best to call a trained HVAC professional. They have the experience and equipment to pinpoint most heater problems in just a matter of minutes.
Here is a word of caution though that you really need to pay attention too. If your HVAC technician does indeed find a problem don’t give them the go-ahead to fix it until they show you a written estimate of what that furnace repair will cost. You also need to tell them if they find anything additional wrong while working they need to talk to you about it before proceeding to fix it.
This will keep you from getting any big surprises when they give you the furnace repair bill.
How to Avoid Being Ripped Off and Get a Fair Price
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Tips On How Not to Be Left Out in the Cold with Your Furnace
The most common time to have your furnace fail to ignite is the first time you go to use it after it has not been used in a while. If you are turning it on it probably means it got pretty cold outside and now you have no heat. There are some things you can do to avoid this scenario and to keep your furnaces ignition in tip-top working order.
Here are some suggestions for doing it:
- Don’t wait until it gets cold to turn on your furnace for the first time. Most people know when the cold season in their area is. It’s recommended that about 1 month before this cold season starts turn on your furnace to check and make sure its running right. If it isn’t you have plenty of time to get it fixed before you need to use it.
- Schedule a routine maintenance furnace check once a year. Again the best time to do this is a month before your areas cold season hits. During this routine maintenance session, your HVAC service person will check over the entire system to make sure it's running OK and clean critical furnace ignition parts. Many times they will catch small problems too and repair them before they become much bigger and more expensive furnace repairs.
These are the common questions we receive regarding a furnace that won’t ignite.
Can you be more specific about how to clean a furnace igniter? Also, is that the same thing as a hot surface igniter?
First, yes, the igniter is commonly called a hot surface igniter or even a glow plug. When they’re getting power, they begin to glow, eventually becoming red-hot, hot enough to ignite gas.
Cleaning your hot surface igniter requires a small screwdriver and fine-grit sandpaper made for metal.
- Turn off power to the furnace.
- Take off the furnace door, and locate the igniter.
- Remove the two screws holding it in place, and gently pull it out with or without detaching the wires.
- Be careful NOT to touch the “stick” part of the igniter. Oil from your hands will burn extra hot and might damage the igniter when it lights.
- Gently sand all sides until the soot has been removed. Re-install and secure it.
Pro Tip: It’s not a bad idea to pre-emptively replace the igniter. If you can clean it, you can replace it. The part is cheap - $8 to $30 for most models – and it can save you from being without heat at some point in the middle of winter due to a failed igniter.
I have a multimeter. Can I test the igniter?
Yes, you can. Great idea, in fact. This furnace troubleshooting step will help you rule out whether the igniter is the cause of the gas furnace not igniting or starting.
Our guide “How to Tell if Your Furnace Ignitor Is Bad & Where to Buy One” has step-by-step instructions that are easy to follow, if you need them. Perhaps if you already own a multimeter, you know how to test an igniter.
The essential steps are to:
- Check if the igniter is getting power. While the igniter is connected, turn the furnace to On.
- When the inducer motor comes on, use the leads on the meter, one on each igniter connection. If it shows about 120 volts, there is power to the igniter. If it is getting power but won’t light, it is bad.
- Check the resistance of the igniter. Switch the meter to Ohms. Good igniters typically read in the 40 to 150 ohms range, or perhaps up to 200. If it reads higher, consider it bad and replace it.
Here’s the full guide. It tells you where to find a new igniter and how to replace it.
I’ve heard about furnace flash codes. What are they, and could they help me solve an ignition problem?
Flash codes are a series of light “blips” using one or two lights. Each furnace’s flash codes are different. They’re also called fault codes.
Some codes definitely relate to furnace ignition problems. These error codes include a bad flame sensor or an electric ignition module.
Take the cover off your furnace and look for one or two lights. They might be steady, which usually isn’t an error code. Or it/they could be flashing in a series such one light flashing twice and the second light flashing once.
Near the lights or on the inside of the cover, there should be a key showing you what the flash codes mean. If there’s no key there, like the one below, then perhaps the manual has it.
Pro Tip: Try to get a new furnace filter in the next 24-48 hours to prevent a lot of dust from coating the blower and other internal parts. If those items get really dusty, use a shop vacuum with a brush head to gently remove the dust.
If you still can’t find a chart showing what the flash codes mean, search online for your furnace’s manual using the model number.
Is there a way to check if a dirty filter is the problem when I don’t have a new one to replace it with? I don’t want to pay for a service call only to find the filter is the problem.
Yes. It’s OK to run a gas furnace for a little while without a filter in place. Remove the filter or filters, turn off power to the furnace, and then turn it back on. This resets the furnace.
Then, turn up your thermostat past the room temperature, and see if the furnace fires up.