5 Reasons Why Your Furnace Keeps Turning On and Off

Have you ever woken up to a cold house in the middle of winter? 

One of the most common service calls I take as a licensed HVAC contractor is for a furnace that won’t stay on. Having a properly operating furnace is essential when the weather is cold. 

Most furnaces today have multiple components that have the potential to fail and can cause your system to work intermittently. The majority of them have LED diagnostic lights on the circuit board that can help direct you to what is failing.

In this article I’m going to cover several of the common reasons your furnace keeps turning on and off, how to determine the cause, and how to find a solution.

Why The Furnace Won’t Stay On?

If you find yourself in a situation where your furnace won’t stay running, don’t fret. Start with simple solutions. A lot of easy troubleshooting can prevent an expensive service call. 

Change your air filter, check your thermostat batteries, and make sure the vents are open first because these are the least involved and easiest to start with. Check for any flashing LED lights on the circuit board in the bottom door of your furnace to see if there’s any diagnostic codes.

Check your pilot light or your hot surface ignitor and reset power to remove any lockouts preventing your furnace from running. If none of these corrects your problem, you may have a more in depth issue that needs to be looked at by a trained professional.

Reason #1: Dirty Flame Sensor

Modern furnaces use a small steel rod called aflame sensor to prove that the burners are lit.

Combustion is typically dirty and sediment can build up over time. If your furnace uses a flame sensor, it will get dirty and stop reading flame effectively, if regular maintenance is not performed yearly. 

The sensor can be easily removed and cleaned with steel wool to remove any build up on the rod. It’s important to use something that won’t score the steel rod.

Coarse materials like sandpaper will leave gouges or grooves that will collect sediment and cause the sensor to get dirty faster than normal.

RELATED: How To Fix Condensation Leaks in Furnaces?

A dirty flame sensor will misread and cause your furnace to stop heating. The burners will light and the flame can’t be proven so the furnace shuts off after ignition. It will usually try this up to 5 times before it locks out and has to be manually reset.

If your gas furnace keeps shutting off and needs to be reset or turns on and off every few seconds, a dirty flame sensor is a likely culprit. Having regular maintenance done annually will help prevent this from happening.

Not only that, but most equipment brands require annual maintenance by a qualified contractor to maintain your equipment’s warranty, so it’s important to have regular maintenance done every year.

How often should I get my furnace serviced?

You should have your furnace serviced every year to make sure it’s running efficiently, extends its life expectancy, and prevents breakdowns.

What are the benefits of getting my furnace serviced?

The benefits of getting your furnace serviced include maintaining equipment warranty, extending service life, and peace of mind knowing your furnace is running as efficiently as possible.

Read Also >> Why The Furnace Cycles On And Off Every 5 Minutes?

Reason #2: Dirty Air Filter

Airflow is important on all furnaces, but it is vital for modern furnaces to operate properly.

Your air filter keeps the air and your equipment clean by removing particulate matter. As debris collects over time, the filter begins to restrict airflow through the furnace.

Heat exchangers in modern furnaces are not as robust as the old cast iron heat exchangers in older furnaces. The metal can’t handle excessive heat and is a thinner material that is more likely to crack.

Temperature safeties, like high limit switches, shut the burner off if the temperature around the heat exchanger gets too high. This is great to help prevent significant damage to your heat exchanger, but something like a dirty filter can cause the limit switch to trip and shut your furnace down.

Usually the burners will fire again once the limit switch cools down and closes and the furnace will continue to heat the house. Most of the time if the limit switch opens multiple times during a run cycle, the circuit board will lock the furnace out for an extended period of time causing the house to drop in temperature.

Replacing your filter is easy and should be done regularly. It’s easier to replace if the blower isn’t running, so turn your thermostat to “off” and allow it to shut down completely. Locate your filter and remove it.

Check the arrow on the new filter so that you orient it correctly. Airflow is toward the furnace, so that’s the direction the arrow should point. Slide the new filter in and turn your system back on. 

filter arrow

How often should I change my air filter?

Most 1″ filters should be checked and changed every month. Larger filters, usually 4″-5″, should be checked every 6 months, but can sometimes last up to a year.

What happens if I don’t change my air filter regularly?

Not changing your filter will cause significantly restricted airflow. Airflow issues will cause your furnace to cycle on and off on high limit trips. It will also damage your heat exchanger and your blower motor and inevitably cause your furnace to fail and you will go without heat.

Reason #3: Faulty Thermostat 

Your thermostat is the switch that tells your furnace when to turn on and when to turn off.

Because of this, it can be a common source of short cycling. If your furnace is kicking on and off before the house has come up to temperature, your thermostat may be faulty.

If your thermostat uses batteries and they’re dying, it may be having a hard time closing the switches telling the furnace when to come on and off. Swapping out the batteries is an easy test to see if your thermostat is the culprit for your furnace issues.

If your thermostat keeps turning off and doesn’t have batteries or is powered by the furnace, it is probably faulty. Like other parts, components wear out and can malfunction over time.

You can jump out the wires at the thermostat to see if it’s faulty. Most gas furnaces use “W” for heat at the thermostat. Using a small piece of wire or using a wire nut, you can jump “R” to “W” to simulate a call from the thermostat.

If your system runs properly, you have a faulty thermostat and will need to replace it.

How can I tell if my thermostat is malfunctioning?

To check your thermostat replace the batteries and jump “R” to “W” to see how your furnace runs. You can also use a digital thermometer to check the accuracy of the temperature reading.

How often should I replace my thermostat?

Thermostats have an average life of about 10 years. 

Read Also >> What Is The Optimal Furnace Temperature of Air From Vents?

Reason #4: Pilot Light or Ignition Problems 

Pilot Light

Older heaters use standing pilot lights to ignite the main burner. 

Because they have a constant flame, thermocouples can fail and sediment can build up. A thermocouple is a safety device that proves to the main valve that the pilot is lit and the valve can safely be opened.

If your pilot keeps going out, there’s a good chance you have a bad thermocouple. It’s also possible the pilot orifice is dirty or clogged causing the pilot flame to be weak and pull away from the thermocouple. The orifice is inside the pilot assembly and can be removed to be cleaned.

Relighting your pilot is a fairly straightforward process. Turn your system off at the thermostat and set your gas valve to “pilot”. Depress the button while set to “pilot” and use a long match or grill lighter to light the pilot gas at the pilot assembly.

Keep the button pressed for 60 seconds and release. If the pilot light stays on, you’re good to go. If it goes out, you probably have a bad thermocouple. If it’s more than that, you may need to call a professional to diagnose it properly and safely.

Hot Surface Ignition

More modern furnaces use hot surface ignition or direct spark ignition to light the burners.

A hot surface ignitor, or HSI, is a small part that sits directly in front of your furnace’s burners. It glows bright orange and it’s temperature usually exceeds 2000°F. Most ignitors are quite fragile and can burn out fairly easily. 

Without this ignition source, the gas cannot light and your furnace will shut down on a failed ignition lockout.

You can remove the wires or plug from your ignitor and use an ohm meter to measure the resistance. Different style ignitors have different ohm ranges, but typically it should be less than 100 ohms.

If it’s reading open or very high resistance, it will need to be replaced.

How often should I check my pilot light?

Your pilot light should be checked every year when the furnace is scheduled for regular maintenance.

What should I do if the pilot won’t stay lit?

If the pilot light won’t stay lit, try replacing the thermocouple. If you can’t do that, or it doesn’t fix the problem, you’ll need to call a professional to properly diagnose the problem and provide you with a solution.

RELATED: How To Restart A Pilot Light On Your Furnace?

Reason #5: Overheating

A furnace can still overheat even if the filter is clean.

If you have too many supply air vents closed, it will cause the limit switch to trip similarly to when you have a dirty air filter.

The blower speed on your furnace may be set too low as well, preventing your heat exchanger from moving heat effectively.

If your gas pressure is too high, your furnace may be over firing. Too much flame will cause your heat exchanger to get too hot, and your blower will not be able to remove the heat fast enough before causing the limit to trip and your furnace to shut down.

High supply air temperature and high temperature rise are the main indicators of this. If the air is too hot coming out of your vents, it can cause the house to be uncomfortable and inevitably cause your furnace to malfunction.

Another reason for overheating is an oversized furnace. If your installing contractor oversized your furnace, it cannot remove the heat it creates effectively and will consistently shut down. Most over sized systems have about half the life expectancy and never really run properly. The only fix for this is to replace the equipment with a correctly sized system.

If your furnace cycles on and off every 5 minutes, it’s probably overheating. Making sure your vents are open and airflow is balanced is very helpful in preventing overheating. If your furnace is over firing, you will need to call a professional to regulate your gas pressure and properly set up your furnace.

What should I do if my furnace is overheating?

If your furnace is overheating start by making sure all the vents are open and checking for proper airflow. If it continues you will need professional help to determine if your furnace is sized properly and if your gas pressure is correct.

How can I tell if my furnace is overheating?

If your furnace keeps turning on and off and doesn’t maintain your thermostat temperature it’s probably overheating. You can also tell by checking the supply air temperature from your vents and if it’s more than 140 degrees, it’s overheating.

Frequently Asked Questions

When To Call A Pro?

If you ever get in over your head trying to troubleshoot or if the problem is more challenging, always call a pro. Gas furnaces are dangerous and if not properly repaired and serviced, you risk damaging your home and injuring yourself or others.

What Is Average Cost of Repair?

The number of different issues and parts can drastically change the cost of repair. Anywhere from $150-$1500, but on average furnace repairs run about $250-$300. Preventative maintenance helps greatly reduce the cost of breakdowns.

Why Does The Furnace Keep Turning Off?

Most of the time your furnace won’t stay on because it needs to be cleaned and maintained. Sometimes it may have a faulty part that needs to be replaced.

What Is The Best Way To Troubleshoot?

Depending on how new your furnace is, check for a window in the bottom door for an LED light. This light can help you identify what the issue is and direct you in how to find a solution.

What Is Most Common Reason For Furnace To Shut Off?

Usually high temperatures cause furnaces to shut down prematurely from poor airflow. Sometimes with high efficiency models, condensate build up will also cause this to happen.

How Can I Fix It?

Start by doing the simple stuff, like changing your air filter and thermostat batteries. Check for LED diagnostic lights and determine what is causing it to shut down. When in doubt call a professional to check things out for you.


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