Why Does My Furnace Short Cycle?

Combustion warnings override troubleshooting.

If a carbon-monoxide alarm sounds, anyone feels ill, you smell gas, or you see soot or flame abnormalities, leave the area and follow local emergency and utility instructions. Do not reset and continue testing.

Write down on-time, off-time, room temperature, and thermostat state.

A repeated two-minute run is different from a long run interrupted by a schedule. Note outdoor temperature, filter condition, open registers, displayed error codes, and whether the thermostat stops calling for heat.

Airflow and control settings are observable without opening equipment.

Check the correct filter and visible returns, keep supply registers open as designed, and verify the thermostat mode and schedule. Never bypass a limit switch, flame sensor, door switch, pressure control, or safety device.

Fuel, combustion, heat exchangers, and mains wiring are professional boundaries.

Share the cycle log and equipment model with the technician. Repeated cycling can involve airflow, controls, sizing, combustion, or safety limits; guessing by replacing parts can hide the real cause.