What usually goes wrong with an A.O. Smith water heater?

First: is it safe to troubleshoot?

If you smell gas near the heater, do not touch the controls or any switch. Leave the house and call the gas utility or 911 from outside. See gas smell near the water heater. Everything below assumes there is no gas smell.

This page is a neutral guide to common symptoms. We are not affiliated with A.O. Smith, and we do not perform repairs.

Read the status light first (gas tanks)

Many A.O. Smith standard gas tanks have a status light on the gas control. A.O. Smith's service handbook for its FVIR models says each code is a set of flashes, then a 3-second pause, then a repeat^[1]. Count a few cycles before you write it down.

Status light	What A.O. Smith's handbook says it means	What it points to
Not lit (0 flashes)	Control off or pilot out	Relight per the label; pro if it will not hold
Solid on	Pilot recently went out and the thermopile is cooling	Turn the knob off and wait 10 minutes before relighting
1 flash	Normal operation	No action needed
2 flashes	Thermopile voltage low	Wiring or thermopile check by a pro
4 flashes	Temperature exceeded, high-limit (ECO) activated	Gas control valve replacement by a pro
5 flashes	Sensor failure	Gas control valve replacement by a pro
7 flashes	Internal control failure	Gas control valve replacement by a pro
8 flashes	Power off failure	Pro diagnosis of the gas control

Source for every row: A.O. Smith's handbook^[1]. Other A.O. Smith models, including power-vent and tankless units, use different displays and codes. Use the chart on your own unit's label or manual.

Two handbook details help homeowners. First, the control has built-in circuitry that requires a 10-minute wait between lighting attempts^[1]. Repeated tries before then will not work. Second, the status light may not blink for up to 90 seconds after the pilot lights, while the thermopile warms up^[1].

Symptoms, likely causes and who fixes them

Symptom	Likely cause	Who fixes it
Pilot lights but will not stay lit	Thermopile, tripped thermal switch, dirty air filter or flame arrestor	Licensed plumber
No hot water, light shows 4, 5 or 7 flashes	Gas control valve/thermostat	Licensed plumber
Burner shuts down and will not relight	Thermal switch tripped by poor combustion, blocked vent or flammable vapors	Licensed plumber, after fumes are cleared
Electric tank, no hot water	Breaker, element, thermostat or high-limit	Homeowner checks breaker; pro for the rest
Water around the base	Relief valve discharge, drain valve, fittings or a failed tank	Pro diagnosis; a leaking tank body means replacement
Rumbling or popping	Sediment on the tank bottom	Flush per the manual, or a pro

The thermal switch. These FVIR tanks have a resettable thermal switch in the combustion chamber. A.O. Smith's handbook says it shuts off gas after high combustion chamber temperatures caused by flammable vapors, a blocked vent, or too little combustion air. It adds that a trip is most likely caused by poor combustion, such as a blocked vent or a dirty filter or flame arrestor^[1]. That is a carbon monoxide question as much as a repair question, so leave it to a pro.

Combustion air. The handbook says sufficient combustion air is critical, and that the base-ring filter and flame arrestor should be inspected and cleaned on each service call^[1]. A homeowner can help by keeping lint, dust, boxes and stored chemicals away from the base.

For more detail on these symptoms, see pilot light won't stay lit, no hot water from a gas heater, no hot water from an electric heater and water heater leaking from the bottom.

What a homeowner can safely check

• Count and write down the status light pattern.
• Confirm the gas valve on the supply pipe is open and other gas appliances work.
• Follow the lighting label once, then wait the full 10 minutes before a second try.
• On electric tanks, check the double breaker. If it trips again, stop.
• Move gasoline, paint and solvents out of the room once you are sure it is safe.

Do not open the burner door, reset the thermal switch, test the thermopile or replace the gas control. Those are gas parts, and the handbook is written for qualified service professionals^[1].

Warranty basics from A.O. Smith's own documents

• When it starts. A.O. Smith's registration page says the limited warranty begins on the date of installation. Without proof of installation, it uses the date of manufacture plus the grace period in the warranty document^[3]. That is why the serial number matters; check it with our age decoder.
• Registration. The same page says registration is voluntary and not registering will not diminish your warranty rights^[3]. Some third-party sites say otherwise. Go by the maker's page.
• Term. On the residential tank warranty we read, the tank and parts terms are set by a warranty code in the model number. That certificate lists 6, 8 and 10 year terms^[2]. Your data plate shows the term for your unit.
• Labor. The owner pays labor, shipping, removal and installation costs, unless state law says otherwise^[2].
• Who is covered. The original owner, while living in the home where it was first installed. The warranty is not transferable^[2].
• Common exclusions include failures from sediment buildup, a closed system without provision for thermal expansion, and a missing relief valve^[2]. Anode rods are excluded as consumable parts^[2].

For service or warranty help, A.O. Smith's warranty lists a toll-free line, 1-800-527-1953, and hotwater.com^[2]. Have the model number, serial number, purchase and install dates ready.

Repair costs and the repair or replace question

HomeGuide's national ranges, published August 2025^[4]:

Repair	Typical range
Thermocouple replacement	$100 to $325
Gas control valve replacement	$150 to $550
Heating element replacement	$100 to $350
Thermostat replacement	$100 to $350
Typical tank repair overall	$150 to $700

Service call fees of $50 to $200 may apply^[4]. These are not quotes.

A.O. Smith says tank water heaters last 8 to 12 years^[5]. HomeGuide suggests considering replacement for a heater over 10 years old, or when the repair costs more than 50% of a replacement^[4]. Because the warranty does not pay labor^[2], an in-warranty gas valve still carries a labor bill.

When to call a pro

• The status light shows 2, 4, 5, 7 or 8 flashes, or stays solid after a 10-minute wait.
• The pilot will not stay lit after one or two attempts.
• The thermal switch has tripped, or you see scorch marks or soot near the burner door.
• Water is leaking from the tank, or an electric tank trips its breaker again.