Furnace LED flashes code 21; inducer may start but burners do not light. Often indicates gas heating lockout or gas valve fault.

Critical safety: If you smell gas, evacuate the area immediately and contact your gas utility. Do not attempt repairs.

• Verify gas supply: Ensure the manual gas shutoff valve near the furnace is in line with the pipe (open). Check that other gas appliances in the home (e.g., stove) are working.
• Inspect gas valve wiring: Turn power OFF. Remove the burner compartment door and locate the gas valve. Confirm the 2-wire or multi-pin harness is firmly connected and not damaged.
• Observe ignition attempts: Restore power and call for heat. Listen for the gas valve click after the ignitor glows. If there is no click and no flame, the valve may not be energizing.
• Reset control: Turn power OFF for 60 seconds to clear lockout, then retry.

Gas valve voltage and manifold pressure must be checked with proper instruments. This should only be done by a qualified technician. If code 21 persists, the gas valve assembly or control board likely needs replacement.