Fix it
Why is my furnace short cycling?
A furnace that fires, runs a few minutes and shuts down again is protecting itself from something — usually overheating or a safety sensor that is not seeing what it expects. Short cycling wears parts fast, so it is worth solving early.
What it usually is
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A clogged filter overheating the heat exchanger
Restricted airflow lets the exchanger overheat, and the high-limit switch cuts the burner. The blower keeps running to cool down, then the cycle repeats. This is the first thing to rule out because it costs a filter.
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A dirty flame sensor dropping the burner
The flame rod proves the burner lit; a film of oxide makes it report no flame and the control shuts gas within seconds. The signature is ignition followed by shutdown in under ten seconds, several times in a row.
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An oversized furnace satisfying the thermostat too fast
A furnace with too much capacity for the house heats the thermostat before the rooms, cycles off, and repeats — short but regular cycles from day one rather than a new development.
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Blocked exhaust or intake tripping the pressure switch
High-efficiency furnaces vent through PVC to an outside wall; snow, leaves or a nest at the terminals makes the pressure switch stop ignition, giving clicks and retries rather than sustained running.
Safety first: If short cycling comes with any smell of gas or exhaust, shut the furnace off at its switch and get it checked before running it again.
What you can check yourself
Replace the filter and make sure every supply register is open. Check the outside intake and exhaust pipes for blockage. If you are comfortable with the panel off, a flame sensor can be cleaned with fine abrasive — power and gas off first. Note the cycle length; under ten seconds points at the sensor, several minutes at airflow.
When it’s a job for a pro
Cycling that survives a clean filter and clear vents means testing the limit switch, pressure switch and control board with a meter — technician territory. Repeated overheating also justifies having the heat exchanger inspected while the panel is open.
A flame-sensor cleaning or limit-switch replacement typically runs $100 to $350; pressure-switch and board repairs $200 to $600. A furnace left short cycling all winter shortens its own life measurably.
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