A. O. Smith 12 Error Code: Loss of flame
Updated October 2026 · Sourced from A. O. Smith documentation
A. O. Smith 12 means different things on different series:
- A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters: The manual lists this as: Loss of flame.
- A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters: The manual lists this as flame failure.
Is it safe to keep running? No. Turn it off and get it checked. Call a licensed service pro. The manual says to consult the manufacturer after the listed checks, and these are gas, electrical or board parts.
12 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| 12 Also: 121 (510 T-D2 model) 110/310: red LED on the computer board blinks three times. Remote controller shows 12. 510 (T-D2): 7-segment display shows 121 | Loss of flame | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters |
| 12 Error code flashes on the temperature controller display | Flame failure | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Note the code and how many times the red LED blinks (110 and 310) or the code on the remote or 7-segment display (510).
- Turn the water heater off and back on once at the power switch or remote controller, then try hot water again.
- Make sure the gas supply valve to the water heater is open.
- Check that the gas supply valve at the water heater is open, as the manual says.
- If you smell gas, do not touch switches, leave and call your gas supplier from outside.
🛑 Stop and call a licensed pro if
- Call a licensed service pro. The manual says to consult the manufacturer after the listed checks, and these are gas, electrical or board parts.
- Call a licensed pro. The manual warns that some remedies involve gas or electricity and need a qualified service technician.
- You smell gas. Leave the house, do not touch switches, and call your gas utility from outside.
- Codes for flame failure, overheating, venting or the control board that return after one reset.
- Water leaking from the heater, the relief valve discharging, or scorching around the burner area.
- A carbon monoxide alarm sounds. Get everyone outside and call 911 or the fire department.
Safe first steps for any A. O. Smith code
- Write down the exact code shown on the controller or the number of status light flashes.
- Turn the water heater power off and back on (or press the reset button the manual describes) once. If the code comes back, stop resetting.
- Make sure the gas supply valve to the heater is open and other gas appliances in the home are working.
- Make sure the cold water supply valve is fully open and hot water taps are not partly closed.
- Look at the outside vent terminal for snow, ice, leaves or anything blocking it (look only).
Sources: A. O. Smith On-Demand Water Heater Installation Manual and Owner's Guide (110, 310, 510 models), Fault Analysis of Error Codes, p. 44; A. O. Smith Direct Vent Tankless Water Heater Operation and Installation Manual (305, 505, 705 Indoor), Error Code Table, p. 9. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
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