Furnace cycles on and off rapidly (every 1–5 minutes) without reaching set temperature; LED shows no fault code.

Steps to check:

• Thermostat location: Ensure the thermostat is away from direct sunlight, supply registers, exterior doors, and heat-generating appliances.
• Adjust thermostat settings: On digital thermostats, increase the cycle rate or temperature differential setting if available. On older mechanical thermostats, adjust the heat anticipator per the furnace control current rating.
• Check for drafts: Seal wall openings behind the thermostat to prevent cold drafts from affecting the sensor.

If rapid cycling persists and the furnace is significantly oversized, consult an HVAC professional about system modifications or staging options.