Furnace Lights Then Goes Out (7 Reasons Why And How To Fix It)

Furnaces are interesting. While under normal circumstances, the heater will light up and remain on without any issues, sometimes a unit functioning usually 30 minutes ago may refuse to light up completely. You may try a thousand times, and it will not come on. In the end, you may be forced to call an HVAC technician to fix the issue.

However, the heater may light up but only stay on for several seconds before going off. And, if you light it again, it will repeat the same pattern – stay on for several seconds or a few minutes, then go off.

If you’re currently experiencing the latter, you may already feel your blood boiling with rage. It can be very frustrating, mainly because the issue often seems to come from nowhere. For example, the heater will be working one minute, and it refuses to stay on the next minute.

Furthermore, it may leave you in icy indoor conditions. And, if it happens at night, you may be forced to weather the cold through the night.  

Knowing potential causes and solutions can help you diagnose the problem and see if you can DIY the repair. However, even if you can’t do the repairs yourself, knowing what could be wrong can make the technician’s work easier and potentially reduce professional repair costs.

Furnace Lights Then Goes Out – Causes and Solutions

The following are a few common reasons why a gas furnace may suddenly refuse to stay on when it’s lit and potential fixes.

1. A dirty flame Sensor

The first suspect is usually the flame sensor. The flame sensor is a relay system that detects the presence of a flame within the furnace. It is a short length of a thin metallic rod that creates a small electric current to confirm a flame in the furnace.

After confirming the flame, it relays the information to the circuit board, which in turn instructs the gas supply valve to continue releasing fuel. If no flame is detected, the circuit board will discontinue fuel supply until the issue is fixed. It does so to prevent furnace fires.

However, the flame sensor can accumulate dirt after some time. When this happens, the sensor may fail to detect a flame, even if there’s one. So, the sensor will return a negative answer. In turn, the circuit board will respond by shutting the gas valve to cut the gas supply, effectively shutting causing the heater to shut off.

Solution: The only solution here is to keep the flame sensor clean. Have an HVAC professional inspect the entire heater and tune it up at least once a year before the heating season begins.

2. Tripped Flame Rollout Switch 

Another potential cause is a flame rollout switch that keeps tripping. The flame rollout switch is a safety mechanism built into gas furnaces to shut down the unit if the gas overheats or starts to burn in areas where it’s not supposed to.

For instance, in most furnaces, it cuts gas supply whenever temperatures inside the furnace exceed 350˚F. It will also cut off the gas supply if the flame has a stopped up vent, a stopped up heat exchanger, or low gas pressure.

Whenever the rollout switches senses danger, it will automatically trip to turn off the furnace until the issue is resolved. Even if you relight the heater, it may only remain on for several seconds before going out.

Solution: You cannot bypass the flame rollout. Don’t even think about it, as it exposes you to extreme risk. However, you can rest the switch by removing the burner access panel to access the switch and switching it back on. But, as we’ve said, don’t do this until the underlying cause is resolved. If the problem doesn’t go away, call an HVAC professional.

3. The Furnace is Short-Cycling 

So far, we’ve assumed that you have an appropriately sized furnace. But, what if you don’t? What if the heater is too big for the application? It happens more often than you’d imagine, especially if you purchase a furnace without the advice of an HVAC professional.

If the heater is too big for the house, you may experience what’s known as short cycling. As the name implies, short cycling means that the furnace is running shorter than ideal. Properly sized heaters complete 3-6 full cycles (on and off) per hour. However, if the furnace is too big for the application, it will reach the thermostat setting faster. This is because heaters cycle off temporarily once the thermostat setting is reached and only resume heating when indoor temperatures drop below the thermostat temperature.

So, a larger heater naturally means you’ll have more frequent off cycles. If the heater keeps going off but comes back on automatically such that indoor temperatures are within a comfortable range, you’re likely experiencing short-cycling.

Solution: The ultimate solution to short cycling is to size the heater correctly. If the problem is already ongoing, you may have to replace the furnace. There’s no other solution.

4. Airflow Issues 

Gas furnaces require plenty of oxygen to run efficiently. Indeed, most gas furnaces are designed only to run when a certain level of oxygen is met. If the airflow in your home is such that the heater cannot draw enough oxygen, the unit will not run.

Remember that lack of airflow can also cause overheating in the furnace. So, many heaters are designed with mechanisms that automatically shut off the burners when there’s insufficient airflow.

Common causes of airflow issues include blocked filters, a broken blower motor, and dirty coils. A dirty filter constricts the flow of air into the furnace, thus “suffocating” the furnace. Dirty coils and a broken blower system can also impact airflow within the furnace.

Another issue is the draft inducer motor located deep within the furnace. The draft inducer is also a blower system. However, its primary purpose is to remove combustion gases from the house and induce the flow of fresh, oxygen-rich air into the unit to facilitate smooth heating. If the draft inducer motor is damaged, the control board will instruct the furnace to shut off.  

Solution: In one word – maintenance. Make sure the furnace is well maintained. Schedule a professional check at least once before the heating season begins. Additionally, always check the unit for dirt buildup and replace the filters are necessary.

Other Potential Issues

Besides flame sensor, flame rollout switch, and airflow issues, the furnace may also abruptly go off almost immediately after you light the furnace for the following reasons;

  • Compromised flue pipe: The flue pipe or exhaust vent helps lead combustion gases out of the house through the roof. If it’s blocked or damaged, the accumulation of combustion gases can cause the furnace to overheat, thus tripping the flame rollout switch.
  • Malfunctioned thermostat: The thermostat is the brain behind furnace operation. It tells the furnace when to run and how fast. If the thermostat is damaged, it may keep coming on and going off. It can also send erroneous values to the furnace. This can cause the furnace to go on and off.
  • Control board issues: Finally, you may also have a malfunctioned or damaged control board. Loose wires, short circuits, or a broken circuit board can cause the furnace to malfunction. The same may happen if the circuit board has dirt buildup.


The furnace can go off unnecessarily for several reasons. However, the most common causes are short cycling, the flame sensor, the flame rollout switch, and restricted airflow. Fortunately, you can rectify all these issues. Always consider professional diagnosis and repair unless you’re very confident in your DIY skills.