Lennox Furnace 12 error code
Twelve flashes mean the control senses a flame signal while the gas valve is not energized. This can indicate a gas valve that is not fully closing or a failed control board.
Also shown as: 12 flashes, twelve flashes, fault code 12, flame without gas valve
Last updated: September 30, 2026
What causes the 12 error?
- Gas valve not closing fully
- Failed ignition control board
- Wiring fault feeding the valve
How to fix Lennox 12 — step by step
Work through these in order, from simplest and safest first. Stop as soon as the code clears.
- Turn the furnace off
Switch the furnace off at the service switch and leave it off. - Check for gas odor
If you smell gas, leave the home immediately and call your gas utility or emergency services from outside.Safety warning: Do not operate switches, phones, or anything that could spark if you suspect a gas leak. - Call a licensed technician
Gas valve and control board diagnosis is strictly professional work.
When to call a technician
- Immediately
- Any gas smell — treat as an emergency
Frequently asked questions
How can there be flame with the valve off?
A worn valve may seep gas, or the board may be misreading the flame circuit. Either way, the furnace must stay off until inspected.
Is this the same as the 5-flash code?
Related but different: 5 flashes is flame sensed with no call for heat (a control fault), while 12 flashes specifically means the valve circuit is de-energized yet flame is sensed.
Sources
Every meaning and fix above was checked against official manufacturer documentation:
FixCode provides general troubleshooting information only — not professional repair advice. If you are unsure about any step, stop and call a qualified technician. Repairs done incorrectly can be dangerous and may void your warranty.