Furnace Shuts Off After 30 Seconds [Fixed]

If your furnace is shutting off after 30 seconds of starting, this is likely du e to a dirty or bad flame sensor. A flame sensor in a furnace is an important device that safeguards you from potential gas leaks. 

It operates by shutting off the furnace within a minute if it fails to detect the flames.

Let’s look at more causes for this problem and the steps you can take to fix the problem.

Reasons a furnace would shut off after 30 minutes of starting

Possible CauseFix
Dirty flame sensorClean or replace flame sensor
Bad flame sensorReplace flame sensor
Dirty Air FilterReplace filter
Defective blower motor / CapacitorReplace capacitor of blower motor
Bad furnace control boardReplace furnace control board
Bad thermostatReplace thermostat

Dirty flame sensor

A flame sensor, as I mentioned earlier, is a safety device that shuts off the gas valve if it cannot detect flames about 30 seconds after turning on the furnace.

However, with age, a flame sensor can become dirty or covered with remains from the combustion of flames.

This buildup of remains on the flame sensor can prevent it from properly detecting the flames, even when the burners are properly lit.

Read also: Carrier Furnace Light Blinking 3 Times [Solved]

This could lead the flame sensor to shut off the furnace by sending a signal to the control board that no flame is present. The control board, in turn, closes the gas valve and that altogether shuts off the furnace.


If your flame sensor is dirty, you can simply try cleaning it to fix the problem. Use a paper bill, such as a $1 bill, wrap it around the flame sensor, and vigorously rub it.  See video below bout how to clean a flame sensor.

Currency notes work pretty well at cleaning the flame sensor, and I don’t know why.


Bad flame sensor

If cleaning the flame sensor does not fix the short cycling problem, you may have a bad flame sensor. In this case, you may need to replace the flame sensor. 

Flame sensors are relatively expensive. Thus, you can try replacing it with a new one before moving to other troubleshooting steps.

The lifespan of a flame sensor is about 5 years, and that of a furnace is about 15-20 years.

 Therefore, you can expect to replace your flame sensor 2-3 times during the lifetime of a furnace.

Dirty Filter

If your furnace is shutting off after 30 seconds of starting, that could be due to a dirty furnace air filter. Do you remember the last time you replaced your air filter?

If it has been more than 2 months, then this is a good time to make a replacement.

Some air handlers and thermostats come with filter replacement indicators. 

However, if you have none of these or you missed the filter replacement message, you can always go back to the furnace to check the status of your filter.

If your furnace filter is dirty, you need to make a replacement immediately to fix the problem. 

Just make sure you buy the correct model by first checking the specifications on your current filter. The location of your air filter may differ depending on the model and make.

 But on many furnace models, it will typically be located near or inside the furnace compartment. Be sure to check your manual for your specific model.

Defective blower motor / Capacitor

A blower motor plays an important role throughout the operations of a furnace.

A furnace blower sucks air from the home via the return registers and forces it over the heat exchangers where it gets heated up and distributed through ductwork.

When the motor is defective, the blower will not circulate the hot air, which can cause overheating.

Overheating in the furnace will trigger the high limit switch and consequently shut off the furnace shortly after starting.

How to know if your blower motor is defective

Open the furnace blower compartment to access the furnace blower, but ensure that you have turned off power to the furnace.

You can try manually spinning the blower wheel. Typically, when the blower motor is bad, it won’t spin. If it does spin manually but is not working during operation, you may have a weak or bad motor capacitor.

Read also: Solid Red Light On Furnace Circuit Board [Solved]

How to know if your motor capacitor is defective:

A capacitor’s job is to store the necessary energy needed to start the blower motor. When the capacitor is weak, it won’t be able to start the blower motor.

One of the symptoms of a bad capacitor is a humming sound from the blower motor when you turn on the furnace.

You will also need a multimeter to verify that the problem is indeed with the capacitor.

The best solution to a bad capacitor is to replace it with a functional one.

In fact, if one of the components of the blower assembly fails, sooner or later, other components are likely to fail as well. Therefore, it’s best to replace the entire blower motor assembly.

Bad furnace control board

If everything else doesn’t appear to be problematic and your furnace keeps shutting off every 30 seconds, you might have a faulty furnace control board.

The furnace control board serves as the motherboard of the furnace and processes instructions from the thermostat.

Read also: Bryant Furnace Code 31 [Fixed]

If you suspect that your control board may be malfunctioning, consider having it replaced. However, before proceeding with a replacement, it’s advisable to seek a second opinion from a local onsite technician.

Bad thermostat

In rare cases, the frequent shutting off of your furnace could be due to a faulty thermostat.

If this is a new installation of your thermostat, try using the previous thermostat and see if the problem goes away.

If it does, then either your new thermostat is faulty, or there might be an issue with the thermostat wiring.

Final thoughts

In conclusion, now that you understand the reasons behind your furnace shutting off after 30 seconds of starting, I hope you have discovered a solution and resolved the issue.

I would also like to emphasize the importance of scheduling annual HVAC tune-ups for your system to prevent unforeseen furnace problems like the one discussed here.

Thank you for reading.

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