74AO Smith · 310 series condensing tankless · Water Heater

AO Smith 310 series condensing tankless error code 74

Remote controller communication fault

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Quick answer

On AO Smith 310 series condensing tankless, 74 is a fault. It means remote controller communication fault. A first thing to check: Wrong model of remote controller fitted, where the unit expects a specific part number.

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: Book a technician
First safe check: Read the model number on the remote and compare it with the part number the installation manual specifies for this unit.

Checked 2026-09-26

What 74 means on this model

The unit and its remote controller are not talking to each other. The appliance will normally still run, so this is usually a comfort fault, but it takes away the set temperature and the fault history you would use to work out what the unit is doing.

Symptoms you will notice

Remote shows no temperature or an error instead of a set point
Set point changes on the remote do not reach the appliance
Error appears on the remote but not on the unit, or the other way round
Remote is dead after a power event or a wiring disturbance
Fault code shows on one device and not the other

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:

Wrong model of remote controller fitted, where the unit expects a specific part number
Loose, corroded, or disconnected wiring between the unit and the remote
Loss of power at the unit or at the remote
A failed remote controller or a failed terminal on the board

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.

Read the model number on the remote and compare it with the part number the installation manual specifies for this unit.
With power off, check the remote wiring connections at both the controller end and the terminal on the control board.
Confirm the unit itself has power at the board, since a remote cannot report a fault the appliance never received.
Note whether the error appears only on the remote, only on the unit, or on both, because that points at which end has failed.

Do not do these

×Do not fit a remote from a different product line just because the connectors look the same. The unit expects a specific controller and the manufacturer calls this out by part number.
×Do not run the remote wiring alongside gas piping or through a wet area where it can be damaged.
×Do not power down the unit repeatedly to clear a communication code. The code clears on its own once the link is restored.

When to call a technician

Worth scheduling, and worth your own checks first

You can usually keep living with this for a short while, but it will not fix itself. Do the safe checks below first, because a tripped switch or a blocked airflow is a five-minute fix that would otherwise cost you a service call.

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.

Reseating a connector and checking the model number is reasonable for a homeowner. Replacing the controller or working at the board is a technician job.

What it costs to fix

Parts

Remote controller $60 to $160. Board terminal or harness $40 to $150.

Labor

Thirty minutes to reseat a connection. One to two hours if a controller or board terminal has to be replaced.

DIY or technician