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

Safety: Turn off power and gas before removing the flame sensor.

Steps to check:

• Clean flame sensor: Remove the flame sensor rod and gently polish with fine steel wool or emery cloth. Wipe clean and reinstall so the rod is directly in the burner flame.
• Check ground connections: Tighten screws securing the burner assembly to the furnace chassis. Ensure there is no heavy rust or paint preventing good grounding.
• Inspect flame pattern: After reassembly, observe the flame. It should be steady and fully cover the flame sensor. If the flame is small or unstable, a technician should check gas pressure and burner condition.