55AO Smith · 310 series condensing tankless · Water Heater

AO Smith 310 series condensing tankless error code 55

Gas valve or valve circuit fault

Stop using it

Quick answer

On AO Smith 310 series condensing tankless, 55 is a fault. It means gas valve or valve circuit fault. A first thing to check: Failed main gas valve or manifold solenoid.

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: Turn the gas supply valve off before inspecting anything near the manifold, and confirm it stays off while you work.

Checked 2026-09-26

What 55 means on this model

The control found an abnormal reading from the gas valve assembly, which covers the main valve, the manifold solenoid and the wiring between them. The unit closes the gas and stops, because a fault in the part that controls gas flow is a hard stop rather than a retry.

Symptoms you will notice

Code appears during startup or on the first demand for hot water
You hear a repeated click or a chattering valve and no flame follows
No hot water at all
Code returns immediately after the power is restored
Unit will not hold a set temperature and drops out repeatedly

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 main gas valve or manifold solenoid
Burnt wiring or a loose connector between the board and the valve assembly
Burn marks on the board where the valve harness plugs in
Supply voltage outside the range the valve accepts
A valve that has been contaminated by the gas supply itself

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.

Turn the gas supply valve off before inspecting anything near the manifold, and confirm it stays off while you work.
With power off at the breaker, unplug the valve harness from the board and look for burnt or green corroded pins.
Look at the board where the harness connects for heat damage or a burnt smell.
Note whether the valve clicks open and then immediately clicks shut, which is the control closing it on a real fault rather than a supply problem.

Do not do these

×Do not cycle the gas valve open and closed repeatedly to make it fire. You are working on the only thing between the gas line and the burner.
×Do not bridge, jumper, or bypass a valve wire to test the burner.
×Do not assume this code is a wiring problem. The valve itself fails often enough that swapping the harness alone will leave you with the same code.

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.

This is licensed gas work with no meaningful homeowner version. The consequences of getting it wrong are a gas leak or an uncommanded burner.

What it costs to fix

Parts

Gas valve assembly $150 to $450. Valve harness $60 to $150. Board $150 to $350.

Labor

Two to three hours including gas isolation, purge, and restart checks.

DIY or technician