Furnace short cycling (starts and stops quickly)

Short cycling is usually a safety opening mid-cycle (limit from low airflow, flame sensor dropping out, pressure switch opening) or a thermostat issue. Read the code and time the cycle.

Single source ~45 min moderateBrands: All

Before you start

If you smell gas: do not operate switches or the furnace, leave the building and call the gas utility from outside.
Make sure the home has a working carbon monoxide (CO) alarm; if it is sounding or anyone has headache/nausea, get everyone out and call 911 / the gas utility.
Turn off power at the furnace service switch or breaker before opening the burner or blower compartment; the door switch alone is not a lockout.
Gas-side diagnosis or repair (gas valve, gas pressure, burners, heat exchanger, venting) is for a licensed gas/HVAC technician only.
Never jumper or bypass a limit, rollout or pressure switch.

Diagnostic steps

Unlock all 4 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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