Furnace lights but shuts off after a few seconds; LED shows 6 flashes on York board.

Safety: Turn off power and gas before servicing burners. If flames appear yellow and lazy, shut down and call a professional.

• Clean flame sensor: Remove the flame sensor and clean the rod with fine steel wool until shiny. Reinstall so the rod is fully in the burner flame.
• Check burner flame: With panels on and power restored, observe through the sight glass. Flames should be mostly blue, stable, and evenly distributed across all burners. If flames lift or blow off, there may be venting or pressure issues.
• Inspect ground connections: Ensure the burner rack and furnace chassis have solid ground connections. Tighten any loose screws or ground wires between the burner assembly and the control board.
• Filter and gas supply: Confirm the air filter is clean and gas shutoff is fully open. If other gas appliances show issues, contact the gas utility.
• Call a technician: If cleaning and grounding do not resolve the 6-flash code, a pro should check gas pressure, manifold pressure, and gas valve operation with proper instruments.