Short cycling means the furnace lights, runs for a few seconds or minutes, shuts off, and then tries again. The house never quite warms up, and the parts wear faster.
Most cases come down to three things: airflow, a safety control, or the thermostat. Here is the order I would check them in, cheapest first.
Safety first. Turn off power at the breaker or furnace switch before you open anything, and shut the gas valve before working near the burners. If you smell gas, leave the house and call the gas company from outside. Never bypass a limit switch, rollout switch or flame sensor.
1. The air filter
A clogged filter starves the furnace of air. The heat exchanger gets too hot, the high limit switch opens, and the burners shut off. A new filter of the right size is the first and cheapest fix. If the limit keeps tripping after that, this guide walks through the airflow checks: High limit switch keeps tripping.
2. The flame sensor
If the burners light and then go out after a few seconds, the flame sensor may not be "seeing" the flame. A thin coat of residue is enough. Often a gentle cleaning fixes it; sometimes a new sensor is the better choice: Flame sensor: clean it or replace it?
3. The thermostat
A thermostat in a drafty spot, near a supply vent, or with weak batteries can call for heat on and off. Check its spot and its settings before you buy parts.
When to stop and call a pro
If the filter is clean, the sensor is clean and the furnace still short cycles, or you see error codes you can't clear, call a licensed technician.
The full walkthrough, with what each symptom looks like, is here: Furnace short cycling: causes and fixes.
Written by the HVAC Fitter team.
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