The furnace starts but shuts down; the LED status light flashes eight times repeatedly.

Safety: Turn off the power and gas before removing burner components. Allow the furnace to cool.

Steps to check:

• Clean the flame sensor: Remove the flame sensor and gently polish it with fine steel wool. Reinstall it so it is fully in the burner flame.
• Check burner ground: Ensure that the burner rack is firmly seated and that mounting screws are tight and free of rust. Clean any corrosion where the burner assembly contacts the furnace chassis.
• Inspect burners: Remove burners if accessible and inspect for rust, debris, or spider webs in the burner ports or orifices. Clean with a soft brush and compressed air (do not enlarge orifices).
• Verify gas supply: Make sure that the manual gas valve is fully open and that other gas appliances operate normally. Low gas pressure requires professional testing with a manometer.

When to call a technician: If the code persists after cleaning and grounding checks, or if you suspect low gas pressure or burner alignment issues, contact a professional.