Burner lights briefly then shuts down; multiple failed ignition attempts before lockout

Safety: Disconnect power and close the gas shutoff valve before servicing.

• Clean flame sensor: Remove the flame sensor from the burner rail. Gently clean the rod with fine steel wool or emery cloth until shiny. Do not sand the ceramic insulator. Reinstall and ensure the sensor is fully in the flame path.
• Check ground connections: Verify the burner assembly and gas valve bracket have a solid metal-to-metal connection to the furnace chassis. Tighten any loose mounting screws and clean off rust or paint that may interrupt grounding.
• Inspect wiring to sensor: Follow the flame sensor lead back to the IVT control board. Ensure the spade connector is tight and the wire insulation is intact.
• Observe flame: Restore power and gas, then watch the burner through the sight glass. If the flame is weak, lifting, or yellow-tipped, shut down the unit and have gas pressure and combustion adjusted by a licensed technician.

If the sensor and grounding are good but the code persists, the IVT main control board may need replacement by a professional.