72AO Smith · 310 series condensing tankless · Water Heater

AO Smith 310 series condensing tankless error code 72

False flame detected

Needs attention soon

Quick answer

On AO Smith 310 series condensing tankless, 72 is a fault. It means false flame detected. A first thing to check: Soot on the flame rod letting it pick up a false signal.

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: Needs attention soon
First safe check: Look at the flame rod through the sight glass and note whether it is clean or coated.

Checked 2026-09-26

What 72 means on this model

The control thought it saw a flame when no burner was commanded and shut down to be safe. The flame rod is usually picking up a stray signal, often from soot, a leaking heat exchanger, or a condensate problem rather than a real flame.

Symptoms you will notice

Code appears while the burner is off
Unit shuts down randomly between uses
You see a glow or a flicker in the burner compartment with no burner running
Water goes cold in the middle of a draw with no other symptom
Code returns after a cleaning visit without anything having changed

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:

Soot on the flame rod letting it pick up a false signal
Heat exchanger leaking, which can let combustion gases reach the rod with the burner off
Condensate drain not installed or blocked on an indoor unit
Wet or contaminated flame rod surface
A failing flame sensing circuit

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 flame rod through the sight glass and note whether it is clean or coated.
On an indoor unit, confirm a condensate drain is fitted at the vent collar and that it is clear and running down to a proper drain.
Look under the heat exchanger for water or a stain that would indicate a leak.
Note whether the unit is installed where it can see a pilot flame, a gas fireplace, or another ignition source that could be picked up by the rod.

Do not do these

×Do not clean the flame rod while the unit is live.
×Do not seal, cap, or relocate the condensate drain. The drain is what keeps water out of the chamber and out of the board compartment.
×Do not continue using the unit to make hot water while ignoring this code, because the underlying cause is combustion related.

When to call a technician

Book a technician, do not keep resetting it

This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.

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.

Checking for a clear condensate drain is a homeowner task. Anything inside the sealed chamber is a licensed technician job.

What it costs to fix

Parts

Flame rod $25 to $70. Condensate drain kit $20 to $60.

Labor

One hour to clean the rod. Two to three hours if the heat exchanger is the source, at which point the appliance is usually replaced.

DIY or technician