AO Smith 310 series condensing tankless error code 12
Loss of flame after ignition
Which models this applies to. This page covers the AO Smith 310 series condensing tankless gas water heater codes.
What is specific to the 310 series condensing tankless platform
Sealed combustion condensing unit available as direct vent, indoor and outdoor versions. The same two-digit fault numbers apply across the versions, and the fault appears on the control board LED and on the remote controller when one is fitted.
Your control board prints the code as text, for example F01 or EE3, so you read it off the display rather than counting blinks. The printed number is the fault itself, not a position in a table.
Quick answer
On AO Smith 310 series condensing tankless, 12 is a fault. It means loss of flame after ignition. A first thing to check: Heat exchanger cracked or leaking, drawing flame into the chamber instead of across the burner ports.
- 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
- Look at the burner through the sight glass during a heat cycle and describe what the flame is doing. A flame that leans, lifts, or goes out when the fan starts is a venting or air problem.
What 12 means on this model
The burner lit and then went out while the unit was heating, three attempts in a row, and the control closed the gas each time. Something is extinguishing a flame that did establish, so the gas supply itself is less likely to be the cause than air supply, a heat exchanger leak, or flame sensing.
Symptoms you will notice
- Code appears while the unit is already heating, not at startup
- Hot water starts warm and goes cold mid-use
- Unit restarts, relights briefly, then drops out again
- Repeated cycling with the fan running between attempts
- Flame visible but not holding, or lifting off the burner
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:
- Heat exchanger cracked or leaking, drawing flame into the chamber instead of across the burner ports
- Insufficient combustion air, so the flame cannot sustain once the burner is running
- Flame rod or air-fuel ratio rod sooted and no longer proving flame
- High limit switch cycling because the fan is not moving enough air
- Loose or broken wiring to the flame sensing circuit
- Excessive vent draft pulling the flame away from the burner
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.
- Look at the burner through the sight glass during a heat cycle and describe what the flame is doing. A flame that leans, lifts, or goes out when the fan starts is a venting or air problem.
- Check for any sign of water on or around the heat exchanger base or the burner compartment floor. On a condensing unit that is condensate, and a steady puddle means the drain is blocked.
- Note whether the fan runs continuously while the burner is lit. On this platform the fan runs for a long time after shutdown as well, so judge it while heating.
- Confirm the vent termination is clear and that intake and exhaust are properly separated.
Do not do these
- ×Do not keep cycling the unit hoping it will hold. A flame that will not stay lit on a sealed combustion appliance is a carbon monoxide condition until proven otherwise.
- ×Do not block, throttle, or modify the induced draft to try to stabilise the flame.
- ×Do not assume a clean air filter or a clean vent is the whole answer. A heat exchanger leak presents the same way and is not something to work around.
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.
Observing the flame and checking the terminal for snow are homeowner tasks. A heat exchanger, the fan, or anything inside the sealed chamber is a licensed technician job.
What it costs to fix
Parts
Flame rod or air-fuel ratio rod $25 to $90. Induced draft fan $180 to $400. Heat exchanger assembly $500 to $1,200 depending on the model.
Labor
One to two hours for a sensing rod or fan. A heat exchanger is a full day of work and usually means replacing the appliance rather than the part.
DIY or technician
Observing the flame and checking the terminal for snow are homeowner tasks. A heat exchanger, the fan, or anything inside the sealed chamber is a licensed technician job.
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
- How this model reports the code
- Printed on the control display
- Models in scope
- AO Smith 310 series condensing tankless
- How this platform differs
- Sealed combustion condensing unit available as direct vent, indoor and outdoor versions. The same two-digit fault numbers apply across the versions, and the fault appears on the control board LED and on the remote controller when one is fitted.
- 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.