Furnace attempts to light but burners do not ignite; LED shows code 34 (ignition proving failure).

Safety: Turn off power and gas before servicing. Allow ignitor to cool; it is fragile.

Steps:

• Verify gas supply: Ensure the manual gas shutoff valve near the furnace is fully open. Confirm gas service is active to the home.
• Inspect hot surface ignitor: Locate the ignitor near the burners. Look for cracks or white spots on the element. Do not touch the element with bare fingers. Replace if damaged.
• Clean flame sensor: Remove the flame sensor rod (usually a single wire to a metal rod in the burner flame). Gently clean the rod with fine steel wool or emery cloth to remove oxidation, then reinstall.
• Check burner orifices: Inspect burner inlets and orifices for spider webs or debris. Clean carefully without enlarging the orifices.

If the ignitor glows but burners never light and gas supply is confirmed, the gas valve or control board may be faulty and should be tested by a technician.