Furnace keeps shutting off

Two patterns, two causes

Time it roughly. The interval matters more than anything else you can observe.

It lights and dies within about ten seconds, repeatedly, then gives up. That is an ignition problem. The board lights the burners, fails to confirm a flame, and shuts the gas off as designed. The flame sensor is the usual culprit, and it is a small repair. See igniter and flame sensor.

It runs for several minutes, blows properly, then stops and restarts later. That is overheating. The furnace trips its high-limit switch because it cannot move enough air across the heat exchanger to carry the heat away. Airflow, in other words.

Those two feel the same in a cold house, and they are different jobs.

The airflow version

• A blocked filter. The first thing to check, and free. Replace it and see.
• Closed or blocked registers. Closing vents in unused rooms starves the system; so does furniture over a return grille.
• A dirty blower wheel. Dust loads onto the blades and the fan stops moving what it should, while the motor sounds normal.
• Collapsed or disconnected ductwork, especially flexible duct under a house.
• A failing blower motor turning slower than it should. See blower not working.

The ignition version

• A dirty flame sensor that can no longer detect the flame.
• A weak igniter that glows but does not get hot enough.
• A pressure switch opening part-way through the cycle, often from a partly blocked vent or intake.
• A gas supply problem, including a partly closed valve.

Stop resetting it

What settles it

A technician watches a full cycle and notes exactly where it stops, then measures: flame sensor current, igniter resistance, pressure switch operation, the limit switch, and the temperature rise across the heat exchanger. Every furnace has a design temperature-rise range on its data plate, and a furnace running above that range is not moving enough air, whatever the filter looks like.