70AO Smith · 310 series condensing tankless · Water Heater

AO Smith 310 series condensing tankless error code 70

Control board fault

Needs attention soon

Quick answer

On AO Smith 310 series condensing tankless, 70 is a fault. It means control board fault. A first thing to check: Failed control board.

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: Power off at the breaker, then look at the board and its connectors for burn marks, discoloration, or a connector that has backed out of its socket.

Checked 2026-09-26

What 70 means on this model

The control reported a fault with itself, usually on its own power-up check rather than in response to anything the appliance measured. A burnt or backed-off connection on the board produces the same code without the board itself being the failed part, so check the connectors before condemning it.

Symptoms you will notice

Code appears the moment power is restored, before any water is used
Unit is completely dead, no fan, no attempt to ignite
No status at all on the remote controller
Code returns after each power cycle
Unit worked before a power event, a storm, or a service visit

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:

Failed control board
Burnt or loose connection at the board
Wiring to the board broken or disconnected
Supply voltage spike or a power event that damaged the board
Moisture or condensate reaching the board compartment

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.

Power off at the breaker, then look at the board and its connectors for burn marks, discoloration, or a connector that has backed out of its socket.
Check that every harness on the board is fully seated before assuming the board has failed.
Look for water staining or standing moisture in the board compartment, which is common on an indoor condensing unit with a blocked drain.
Confirm the appliance is on a circuit that is not shared with a swamp cooler, a well pump, or a workshop compressor, all of which are common sources of voltage spikes.

Do not do these

×Do not spray any cleaner on the board and do not wipe it with a wet cloth. It is live electronics in a damp environment.
×Do not swap boards without checking the wiring first, because a harness problem will damage the replacement too.
×Do not leave the unit in service with water standing in the board compartment. That is how the next board dies as well.

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.

A technician should do this. The board is mains-connected, sits in a wet environment, and the diagnosis is usually the hard part rather than the swap.

What it costs to fix

Parts

Control board $150 to $350. Replacement is usually the whole part, not a component level repair.

Labor

One to two hours to diagnose and replace, more on a unit where the board is in a sealed compartment.

DIY or technician