Furnace runs but produces no heat, supply air is cold or only slightly warm

Safety first: If you smell gas or see signs of fuel leakage, shut off the supply, evacuate, and contact your gas/oil supplier. Do not attempt to light the burner manually.

Steps:

• Check thermostat and mode: Ensure the thermostat is set to HEAT and above room temperature.
• Verify fuel supply: Confirm that the main gas cock or oil valve is open and that the fuel tank is not empty.
• Check burner control status: Observe the Danfoss burner control module. A steady or flashing fault LED often indicates lockout. Press the reset button once only. If the burner does not start or locks out again, do not continue resetting.
• Inspect manual reset limits: With power off, locate any manual reset high limits near the burner. If tripped, press to reset and investigate airflow or overheating causes.
• Call a technician: If the burner still does not fire, a professional should test the ignition transformer, flame sensor, and fuel delivery components.