Furnace LED shows code 21 (gas heating lockout), no heat, blower may run for a short time then stop.

Safety first: Turn off power and gas before servicing. Repeated lockouts indicate a combustion or gas control problem.

• Clean flame sensor: Locate the flame sensor rod on the opposite side of the ignitor at the burner assembly. Remove the mounting screw and gently clean the rod with fine steel wool or a Scotch-Brite pad until shiny. Do not sand heavily or bend the rod. Reinstall firmly.
• Inspect burner flame: After restoring power and gas, call for heat and observe the flame. It should be blue and stable, with minimal yellow tipping. Uneven or lifting flames can indicate dirty burners or incorrect gas pressure.
• Check ground connections: Ensure the burner rack and control board ground wires are tight to the furnace chassis. Poor grounding can prevent proper flame sensing.
• Reset power: Turn the furnace power off for 30 seconds, then back on to clear the lockout. If the code 21 returns after several attempts, the gas valve or control board may need replacement.

Do not continue resetting a furnace that fails to light properly; have a licensed technician check gas pressure and combustion safety.