Furnace stops and LED shows code 21 indicating gas heating lockout (often after multiple failed ignition attempts).
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Safety warning: Repeated ignition failures can lead to unburned gas accumulation. If you smell gas, evacuate and contact your gas utility immediately.
- Power cycle to clear lockout: Turn off furnace power for at least 60 seconds, then restore. This clears the lockout.
- Observe ignition sequence: Call for heat and watch: inducer starts, ignitor glows, gas valve clicks, burners light. Note at which step the sequence fails.
- Check gas shutoff valves: Ensure the manual gas shutoff at the furnace and any upstream valves are fully open.
- Inspect condensate (condensing models): A blocked condensate trap or drain can prevent proper pressure switch operation and cause lockouts. Clean the trap and ensure hoses are not kinked.
If the furnace repeatedly enters lockout after basic checks, the gas valve, ignition control, or combustion air system needs professional diagnosis with a manometer and combustion analyzer.