Furnace starts and runs but shuts off before reaching thermostat setpoint; frequent short cycles.

Safety first: Turn off power before accessing internal components.

• Replace filter: Install a clean filter and ensure it is the correct size and orientation.
• Check supply and return airflow: Open all registers and ensure returns are not blocked. Remove any obstructions from grilles.
• Thermostat location: Make sure the thermostat is not directly above a supply register, near a space heater, or in direct sunlight. Relocation may be necessary.
• Observe LED codes: Watch for any limit-related codes when the furnace shuts off; this indicates overheating and airflow issues.

If short cycling persists with good airflow and proper thermostat placement, a technician should evaluate furnace sizing and limit switch operation.