10AO Smith · 310 series condensing tankless · Water Heater

AO Smith 310 series condensing tankless error code 10

Warning for the combustion lockout

Stop using it

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, 10 is a fault. It means warning for the combustion lockout. A first thing to check: Snow, debris, a bird nest, or a stuck damper blocking the intake or exhaust terminal.

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: From outside, look at both the intake and the exhaust terminals for snow, debris, nests, or a flap that is stuck open.

Checked 2026-09-26

What 10 means on this model

This is the warning stage, not the shutdown. The control has measured something wrong with the air moving through the unit and it expects the combustion lockout code to follow if the condition is not cleared. Treat it as the last cheap opportunity to fix an air supply or venting problem before the unit stops making hot water.

Symptoms you will notice

Code appears shortly after the unit starts heating
Unit locks out and will not stay in, or resets and drops out again within a minute
Hot water goes cold while the unit is running
Unit installed in a closet, a small utility room, an unvented attic, or near a snow-covered termination
A damper in the vent system is stuck, or vent flaps were fitted at the termination

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:

Snow, debris, a bird nest, or a stuck damper blocking the intake or exhaust terminal
Vent termination too close to the unit so the unit is drawing its own exhaust back in
Appliance in a confined space with no make-up air opening
Vent run longer than the manual allows, or fitted with more elbows than the manual allows
Grease and dust buildup on the burner and the induced draft fan
A unit installed at elevation with the altitude function not configured

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.

From outside, look at both the intake and the exhaust terminals for snow, debris, nests, or a flap that is stuck open.
Confirm the separation between the intake and the exhaust terminals matches the termination clearances in the installation manual.
Confirm the room the unit is in has an opening that lets replacement air in, and that the unit is not in a closed closet.
Count the elbows in the vent run and measure the total length against the manual limits. For this platform the run must stay under 50 feet with fewer than five elbows.
If the unit is new in a space it was not originally in, have the whole venting re-checked before putting it back in service.

Do not do these

×Do not remove, modify, or tape a vent terminal, a damper, or a vent flap to clear this code.
×Do not relocate or resize the combustion air opening. The opening size is part of the certified venting arrangement.
×Do not keep resetting the unit and leaving it. This code is the warning stage; the following code is a full combustion lockout and a carbon monoxide shutdown.
×Do not run the unit in a space you have sealed up for energy reasons without re-checking the air supply.

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 snow and debris from a terminal is a homeowner task. Anything involving the vent run, the damper, the air openings, or the fan is licensed work, because getting it wrong is a carbon monoxide exposure.

What it costs to fix

Parts

Damper or terminal parts $30 to $120. Induced draft fan $180 to $400 if it is seized rather than blocked.

Labor

One to two hours to clear a terminal or correct a vent run. Venting rework runs $300 to $900 depending on how the unit is routed.

DIY or technician

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: 2026-09-26
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.