Furnace short cycling wastes fuel and wears out parts. The most common causes - dirty flame sensor, restricted airflow, blocked flue, oversized furnace - explained.
Your furnace fires, runs for 30 seconds to a few minutes, shuts down, then tries again. Each start is the hardest moment on the ignition system and blower, so short cycling burns through components fast.
The most common cause by far. A carbon-coated sensor cannot prove flame, so the control board closes the gas valve as a safety measure. Cleaning or replacing it is inexpensive.
A clogged filter or closed registers cause heat to build inside the cabinet until the high-limit switch trips. Symptom: it runs longer than a flame-sensor fault but still shuts off early.
Snow, nests, or a failed inducer motor prevent proper venting. The pressure switch then refuses to allow ignition — a genuine CO safety feature doing its job.
An oversized unit satisfies the thermostat too quickly, cycling constantly. This one cannot be repaired — only corrected at replacement with a proper load calculation.
A thermostat in direct sun or above a supply register reads the wrong temperature and cycles the system needlessly.
Guesswork costs more than a service call. Talk to a licensed technician now.
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