Why does my furnace turn on then off right away?

Author: Caroline Parker  |  Last update: Wednesday, July 9, 2025

These could include a dirty filter, blocked vent, or malfunctioning flame sensor. You can switch for a clean furnace filter, or have your HVAC technician resolve a more complicated issue. If you can't diagnose why your furnace is short cycling, you should check with a trusted local technician.

Why does my furnace kick on but not stay on?

Insufficient airflow into the furnace

For the burners to ignite, they need airflow. The airflow into the furnace comes from the blower fan that draws air through the return air ducts. But if this airflow is choked off because of a clogged furnace filter or faulty blower, the burners may not be able to stay on for long.

Why does my furnace kick on and then kick off?

The top causes to blame when a furnace kicks on and off are: The air filter needs changed. The thermostat batteries need to be replaced. Blower motor malfunctions.

Why does my furnace ignite then turn off immediately?

Having A Furnace Bad Flame Sensor

A furnace with a bad flame sensor means that the flame will shut off immediately after it ignites. The sensor's job is to monitor the gas valve. It acts as a safety precaution so that raw gas doesn't get into your home. The sensor switches off when it doesn't detect a flame.

How do I stop my furnace from short cycling?

The simple solution: Turn your furnace off and check the filter. If it's dirty or it's been 3 months or more since you changed it, put a new, high-quality filter in place.

Furnace turns on then turns off after seconds - Common Causes

Is short cycling bad for a furnace?

When a furnace is short cycling, it's working hard to produce heat but isn't making any, causing strain on your furnace. If left unchecked, this could cause major issues for your furnace resulting in hefty repairs.

Why does my furnace run for a short time then shut off?

If your furnace shuts off after a few minutes and starts more than eight times an hour, it may require repair. Common causes of furnace short cycling include restricted airflow, a malfunctioning flame sensor, or a poorly placed thermostat.

Why is my furnace igniting but not staying lit?

Dirty flame sensor

If this sensor is dirty, the dirt can cause the sensor to operate intermittently. When this happens, your furnace will most likely light, but because the sensor can't properly detect if a flame is present. The sensor may signal the ignition cycle to end, which your furnace won't stay lit.

How to tell if a flame sensor is bad?

Now that you know why a flame sensor is necessary, consider these warning signs of a bad flame sensor in your furnace.
  1. Flue Line Corrosion. ...
  2. High Energy Bills. ...
  3. Yellow Flames. ...
  4. Furnace Short Cycling. ...
  5. Dirt on the Sensor. ...
  6. Visible Damage on the Flame Sensor. ...
  7. Age.

Why does my furnace only stay lit for a few seconds?

If the burners visibly ignite, but then go out after approximately 6 seconds, this may indicate a dirty flame sensor. Multiple failed ignition attempts will typically generate an "Ignition Failure" diagnostic on the ignition control board.

How to tell if a thermostat is bad?

A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.

Why does my furnace shut off after 30 seconds?

1- Furnace Keeps Shutting Off After a Few Seconds Due to Rust. If your furnace keeps turning off before reaching the intended temperature, the culprits are likely rust and corrosion. Rust is a result of invasive water damage like corrosion, humidity, or leaks in your furnace system.

Why does my furnace turn on then off then on again?

Check Your Flame Sensor

If this becomes dirty or corroded, it can cause short cycling and result in your furnace blower shutting off and on frequently. You can clean the flame sensor yourself with a rag or brush and some rubbing alcohol, but if it doesn't work, you may need to call a professional for help.

What is a hard reset on a furnace?

The purpose of the reset button on a furnace is to shut it down before it can trip your circuit breaker. The need for this may arise if the furnace becomes too hot, or if there is some sort of electrical system interruption.

Why is my furnace not turning on with the thermostat?

The thermostat may have a dead battery, so replace them with fresh ones and try again. Your furnace may turn on right away after this. If it doesn't, you may have a faulty power circuit. Have an HVAC professional diagnose this issue, and provide you with professional recommendations.

How to reset furnace flame sensor?

There is a work-around to waiting. If there is no safety risk (like a gas leak), a furnace can be reset to bypass this wait. Turn off the furnace's power switch and wait 10 seconds before turning it back on.

How much does it cost to replace a flame sensor?

Most furnace flame sensors cost less than $20, but if you opt to have a professional HVAC technician make the repair, you can expect to pay between $75 and $250. Need help replacing your flame sensor?

Why is my furnace short cycling?

Furnaces have a safety feature called a limit switch. If it senses the furnace is overheating, it will cause the furnace to shut down. If the limit switch is faulty, it can think the furnace is overheating and shut it down prematurely, causing short cycling. If it fails, this can cause repeated overheating.

Why does my furnace keep igniting then shutting off?

A dirty or corroded flame sensor can cause your furnace to keep shutting off unexpectedly. The flame sensor detects if the furnace's burner is lit, and if it doesn't register the flame, it will shut off the gas valve to prevent a gas leak.

Why is my furnace starting but not staying on?

Insufficient airflow or insufficient gas pressure. A defective, malfunctioning ignition board, flame sensor, light switch, thermostat, thermocouple, fan limiter, or draft inducer fan. The drainage line is blocked, resulting in a full condensate pan. Temperatures are so low that the furnace cannot stay lit.

How do you know if you have a bad flame sensor?

Short cycling is a common sign of a bad flame sensor. The sensor thinks that there isn't a flame, so it shuts off the gas as a safety mechanism. Except, there is a flame and without the gas to supply it, it dies. This is a frustrating problem, but an HVAC technician can fix it.

How do I know if my thermostat is bad?

7 Ways to Tell Your Thermostat Is Broken
  1. The Thermostat Has No Power. ...
  2. The AC or Furnace Won't Turn On. ...
  3. The Heater or AC Won't Turn Off. ...
  4. The Thermostat Reading Doesn't Match the Room Temperature. ...
  5. The Thermostat Doesn't Respond. ...
  6. The HVAC System Is Short Cycling. ...
  7. The Thermostat Forgets Programmed Settings.

How to check furnace filter?

Checking & Changing a Furnace Air Filter

Sometimes instructions are printed on the furnace itself. Turn off your furnace and slide the filter out. Then, hold it up to the light. If you can't see the light shining through, it's dirty and needs to be cleaned or replaced.

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