Furnace starts then shuts off after a few seconds; repeated short cycling without heating the home

Safety: Turn off power and gas before internal cleaning.

• Clean flame sensor: Remove the ionization electrode and clean the metal rod with fine abrasive pad. Reinstall securely.
• Check filters and flow: For hydronic systems, ensure all valves are open and radiators are not air-locked. For warm-air systems, check return air filters and supply registers.
• High-limit switch: Inspect wiring to the high-limit thermostat. If the switch is manually resettable, follow the manual to reset once. If it trips again, find and fix the cause rather than bypassing it.

Persistent short cycling should be evaluated by a professional to prevent heat exchanger damage.