Furnace lights but shuts down after a few seconds; LED flashes 5 times repeatedly; you may hear the ignitor click multiple times.

Safety: Turn off power and gas before removing the burner compartment cover.

Steps:

• Locate the flame sensor (usually a single rod with one wire, mounted at the burner assembly). Remove the mounting screw and gently pull out the sensor.
• Clean the metal rod with fine steel wool or a Scotch-Brite pad until shiny. Do not use sandpaper that leaves deep scratches. Wipe with a clean dry cloth and reinstall, ensuring it is positioned directly in the burner flame.
• Inspect the burner assembly for rust, debris, or misalignment. Clean burner ports with a soft brush and compressed air (outdoors) if necessary.
• Check the ground wire from the burner rack to the furnace chassis and from the control board to ground. Tighten any loose screws.
• Restore power and gas, then run a heat cycle. If the code persists, the flame sensor or control board flame-sensing circuit may need replacement.

Warning: If you suspect low gas pressure or irregular flame, contact your gas utility or HVAC technician; do not attempt to adjust gas pressure yourself.