American Standard · Furnace / HVAC
12 Flame sensed with gas valve closed
What this code means
The board is seeing a flame signal even though it never opened the gas valve — a safety-critical fault that locks the system out.
Most likely causes
- Stuck-open gas valve
- Cracked flame sensor sending a false signal
- Control board relay failure
What to try first
Do not attempt to bypass this fault. Shut off the gas supply at the shutoff valve and stop using the furnace until inspected.
Safety first: stop using the appliance and shut off its gas or power until it has been inspected — this fault category is associated with fire or gas risk.
FAQ
What does American Standard error code 12 mean?
Flame sensed with gas valve closed. The board is seeing a flame signal even though it never opened the gas valve — a safety-critical fault that locks the system out.
Can I fix American Standard code 12 myself?
No — stop using the appliance and shut off its gas or power until it has been inspected. This fault category is associated with fire or gas risk.
What causes American Standard code 12?
The most common causes are: stuck-open gas valve; cracked flame sensor sending a false signal; control board relay failure.
Other American Standard furnace codes
All American Standard furnace error codes →
Other furnace brands: Trane · Carrier · Lennox · Goodman · Amana · Rheem · Bryant · Generic · Nest
This information may contain errors or not apply to your exact model — always double-check it against your appliance's own manual before acting. If there's any chance of a gas, fire, electrical, carbon monoxide, or other safety hazard, stop and call a licensed professional or the appropriate agency (e.g. your gas utility) instead of relying on this page alone.