Burners light but go out after a few seconds; furnace retries several times then locks out.

Safety first: Turn off power and gas before removing the burner door or flame sensor.

• Clean flame sensor: Remove the flame sensor rod from the burner assembly. Gently polish the rod with fine steel wool or emery cloth until shiny. Wipe with a clean dry cloth and reinstall.
• Check wiring: Ensure the flame sensor wire is firmly connected at both the sensor and the control board.
• Inspect burner flame: After reassembly, restore power and gas. Call for heat and observe the flame. It should be blue and stable, touching the flame sensor.
• Verify ground: Make sure the furnace cabinet and burner assembly have a solid ground connection to the electrical system.

If cleaning does not resolve the issue, the sensor or control board flame-sensing circuit may be defective and require replacement.