A. O. Smith 10 Error Code: Warning for the 991 code (air or exhaust problem)
Updated October 2026 · Sourced from A. O. Smith documentation
A. O. Smith 10 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: Warning for the "991" error code.
- A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters: The manual lists this as an air supply or exhaust blockage.
Is it safe to keep running? Check the steps below first. If the code stays or turns into 99, turn the heater off and call a licensed pro to check gas type, venting, altitude and burner.
10 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| 10 Also: 101 (510 T-D2 model) 110/310: red LED on the computer board blinks five times. Remote controller shows 10. 510 (T-D2): 7-segment display shows 101 | Warning for the 991 code (air or exhaust problem) | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters |
| 10 Error code flashes on the temperature controller display | Air supply or exhaust blockage | 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.
- Look at the outdoor air intake and exhaust terminals for blockage (visual check only).
- Check there is clear space around the unit and terminals. Do not open the vent pipes.
- Look at the flue inlet and exhaust terminals for blockage (visual check only).
- Do not open or rework the vent pipes yourself.
🛑 Stop and call a licensed pro if
- If the code stays or turns into 99, turn the heater off and call a licensed pro to check gas type, venting, altitude and burner.
- Call a licensed pro for vent materials, connections and vent length, which the manual lists as checks.
- 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.
Ryan Giles · Red Seal Certified Plumber · Ask a plumber · Ask an HVAC technician