A. O. Smith 99 Error Code: Imperfect combustion
Updated October 2026 · Sourced from A. O. Smith documentation
🧭 Check a few things first
The manual lists this as: Imperfect combustion.
Is it safe to keep running? Check the steps below first. Turn the heater off and call a licensed pro if it returns. The manual lists gas type, altitude, venting and a dirty burner or fan as things to inspect.
99 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| 99 Also: 991 (510 T-D2 model) 110/310: red LED on the computer board blinks five times. Remote controller shows 99. 510 (T-D2): 7-segment display shows 991 | Imperfect combustion | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) 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).
- Turn the heater off and back on once after clearing any visible blockage.
🛑 Stop and call a licensed pro if
- Turn the heater off and call a licensed pro if it returns. The manual lists gas type, altitude, venting and a dirty burner or fan as things to inspect.
- 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).
Source: A. O. Smith On-Demand Water Heater Installation Manual and Owner's Guide (110, 310, 510 models), Fault Analysis of Error Codes, p. 45. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
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Related A. O. Smith codes
03 : Incorrect DIP switch setting10 : Warning for the 991 code (air or exhaust problem)11 : Ignition failure12 : Loss of flame31 : Outlet thermistor failure32 : Inlet thermistor failure61 : Fan motor fault72 : False flame detection74 : Remote controller communication error02 : No burner operation in freeze protection mode14 : Thermal fuse16 : Over temperature warning