The furnace lights, then shuts off a minute later

Why it will not settle down on its own

Every one of those shutdowns is the furnace stopping itself on purpose: a safety has opened, or the flame signal has fallen away. Nothing about that resolves by repetition.

It also gets worse. Each cycle is another ignition on the same ignitor, and after a set number of attempts the control stops trying altogether. A furnace short cycling on Friday is often a furnace with no heat at all by Sunday.

One check is worth making, because it is not a repair. A filter left in since spring can be loaded enough that the blower cannot pull air through it. The heat exchanger overheats, the limit switch opens, and the burner drops out — exactly this pattern.

What the visit involves

A technician watches a full cycle with a meter on the flame sensor: a signal that starts strong and falls away within seconds points at the sensor rather than the gas valve. Then the limit, the pressure switch and the flue, and a temperature rise measured across the heat exchanger — a rise above what the data plate allows is an airflow problem.

One split matters more than the rest. A furnace dropping out because the limit keeps opening is a different job from one dropping out because the heat exchanger has cracked, and the second ends with the gas being shut off.

What to say when you call

• How long it runs before it quits, in seconds or minutes
• Whether the blower keeps running after the burner goes out
• When the filter was last changed
• Install year, brand, your address and ZIP code

Common questions

Is it harmful to let it keep trying? It is hard on the ignitor, and a control that has counted its failed attempts will lock out. The heat stops being intermittent and starts being absent.

Could it really just be the filter? It can, often enough to be worth changing one before the call. If the pattern is unchanged on a clean filter, the limit is opening for another reason.

Why does it behave in mild weather? On a mild evening the furnace reaches the thermostat setting before the fault has time to show. The colder it gets, the longer each cycle runs, and the sooner whatever is opening gets to open.