AO Smith 310 series condensing tankless error code 99
Incomplete combustion, unit locked out
Quick answer
On AO Smith 310 series condensing tankless, 99 is a fault. It means incomplete combustion, unit locked out. A first thing to check: Combustion air or exhaust restricted, which is what the warning code reports.
- Applies to
- AO Smith 310 series condensing tankless
- AO Smith 310 series condensing tankless gas water heater codes · Water Heater
- Type of code
- Fault
- Urgency: Stop using it
- First safe check
- Stop using the appliance for hot water until the cause is found. This is the code that exists to keep combustion gases out of the building.
What 99 means on this model
This is the lockout the warning code points to. The control measured exhaust it could not accept, decided the burner was not running cleanly, and shut the gas off. On a sealed combustion unit this is the carbon monoxide protection working, and the cause is always upstream of it, in air supply, a heat exchanger leak, or a dirty burner.
Symptoms you will notice
- Unit stops heating and stays stopped until the breaker is cycled
- A warning code appears first, then this one
- No hot water after the unit has been running normally
- Any smell of exhaust, a sulphur smell, or a headache after running the unit
- Flame through the sight glass looks yellow, orange, or sooty
- Shower or exhaust fans in the room make the condition worse
Common causes
The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:
- Combustion air or exhaust restricted, which is what the warning code reports
- Heat exchanger cracked or leaking
- Dirty or blocked burner nozzles and a dusty induced draft fan
- Soot on the flame rod or the air-fuel ratio rod
- Incorrect gas type or an altitude setting that does not match the site
- A vent run that exceeds the length and elbow limits, or a blocked terminal
What you can safely check
These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.
- Stop using the appliance for hot water until the cause is found. This is the code that exists to keep combustion gases out of the building.
- If anyone in the home has a headache, nausea, or dizziness that eases when they leave, leave the building and call emergency services or the gas emergency number.
- Install or check a working carbon monoxide alarm on every level of the home, and confirm the one near the appliance is the alarm.
- From outside, clear snow and debris from both the intake and exhaust terminals, then look for a stuck damper or a flap fitted at the termination.
- Confirm the room has a make-up air opening and that the unit is not in a closed closet or a sealed utility space.
Do not do these
- ×Do not reset the unit and leave it. This lockout will clear temporarily and come back, and each return means another incomplete burn.
- ×Do not run the unit with a window cracked or a fan blowing to vent the room. That treats the symptom and leaves the carbon monoxide risk in place.
- ×Do not relight the burner manually. The unit is in a designed shutdown and manual lighting bypasses the exact protection that stopped it.
- ×Do not use the unit until a licensed technician has found the cause and confirmed the burn is clean.
When to call a technician
Stop using the appliance and call today
This one is a safety limit, not a nuisance. The control is shutting the unit down to prevent damage or a hazard, and it will keep doing that until the underlying cause is fixed. Running it anyway risks the expensive failure it was designed to prevent.
- This is a fuel-burning appliance, so treat anything involving a burner, gas valve or flue as technician work. If you smell gas at any point, leave the property and call the gas utility from outside rather than continuing to diagnose.
- If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.
Clearing a terminal and confirming the air supply is a homeowner task. Everything inside the sealed chamber, and anything touching gas or venting, is a licensed technician job with carbon monoxide consequences if it is wrong.
What it costs to fix
Parts
Induced draft fan $180 to $400. Air-fuel ratio rod $60 to $160. Burner assembly $200 to $500. Heat exchanger $500 to $1,200.
Labor
Two to four hours to clear a terminal, correct a vent run, or clean the burner. Venting rework $300 to $900. A heat exchanger is usually a replacement, not a repair.
DIY or technician
Clearing a terminal and confirming the air supply is a homeowner task. Everything inside the sealed chamber, and anything touching gas or venting, is a licensed technician job with carbon monoxide consequences if it is wrong.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- AO Smith 310 series condensing tankless gas water heater codes
- Models in scope
- AO Smith 310 series condensing tankless
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.