On most AO Smith residential gas tank water heaters, a single red LED on the Honeywell gas control valve reports the unit's condition by flashing a count rather than a numeric code: one flash every three seconds means normal operation, and any other pattern points to a specific fault printed right on the control's face.
Hot Water Brief is an independent cost-research site. We are not a contractor and do not perform the work ourselves.
AO Smith's FVIR service handbook notes the control uses a thermopile, not a thermocouple, and that the light can lag the pilot by up to 90 seconds, with a 10-minute wait required between relighting attempts.
| Flashes | What it means | What to check |
|---|---|---|
| 0 (LED not lit) | Control off, pilot out | Relight; check the thermal switch (TCO) |
| Solid, not flashing | Pilot out, thermopile cooling | Wait 10 minutes, then relight |
| 1, every 3 seconds | Normal operation | None |
| 2 | Thermopile voltage low | Replace if output reads under 350 mV DC |
| 4 | Temperature exceeded, ECO tripped | Replace the control |
| 5 | Sensor failure | Replace the control |
| 7 | Internal control failure | Replace the control |
| 8 | Power off failure | Replace if the pilot stays lit with the knob off |
Source: A.O. Smith, Service Handbook: Residential FVIR Gas Water Heaters, form 327726-001, May 2014.
AO Smith's tankless line reports faults as a 3-digit numeric code instead of a flash count. The full code shows on an optional remote control's digital display, while the water heater's own 7-segment LED cycles through the same digits one at a time, pausing, then repeating until the fault clears.
AO Smith's manual for its 910 and T-M50 tankless series lists more than 20 such codes, and a linked multi-unit system adds two digits to identify which unit is at fault.
| Code | What it reports |
|---|---|
| 111 | Ignition failure |
| 121 | Loss of flame |
| 311 | Output thermistor failure |
| 321 | Inlet thermistor failure |
| 441 | Flow sensor failure |
| 510 | Abnormal main gas valve |
| 611 | Fan motor fault |
| 991 | Imperfect combustion |
Source: A.O. Smith, 910/910 ASME (T-M50/T-M50 ASME) Installation Manual and Owner's Guide, Troubleshooting, Error Codes.
An AO Smith tank tends to fail at the pilot and thermopile assembly, the resettable thermal switch, or from sediment settling in the bottom of the tank, while an AO Smith tankless unit tends to fail at a thermistor, the flow sensor or the fan motor, the same parts its own codes are built to flag.
AO Smith's FVIR handbook traces most thermal-switch trips to a blocked vent or a dirty base-ring filter, and lists sediment buildup as a routine reducer of hot water volume in an older or hard-water-fed AO Smith hot water heater. Its tankless manual ties thermistor faults to the codes above, and traces a hot-side-only pressure drop to a melted heat trap, most often from soldering pipe while still connected.
For symptom-by-symptom detail across brands, the general water heater problems and repair cost page covers cause and typical repair range.
Coverage depends on the model line printed on the data plate. AO Smith's standard residential tank warranty runs 6 to 10 years on the tank and parts depending on its warranty code, and its HX3 gas tankless models carry 15 years on the heat exchanger and 5 on parts, but labor is the owner's expense past the first year.
AO Smith sets the tank warranty term by the code on the model number, shown below. Labor is the owner's expense except where state law differs, and anode rods are excluded entirely as consumable maintenance parts.
| Tank warranty code | Tank and parts term |
|---|---|
| E, F or G | 6 years |
| H | 8 years |
| P, X or PX | 10 years |
Source: A.O. Smith, Limited Warranty, Residential Type Water Heater, form 100293072 Rev. C, April 2018.
Tankless warranties split further by model line. HX3 gas models carry 15 years on the heat exchanger and 5 on parts residentially, 1 year commercially, and cover scale-related heat exchanger failure through AO Smith's X3 scale prevention technology. The 910/T-M50 series matches that 15-year term only with proper recirculation pump control, otherwise dropping to 3 years, capped at 9,000 run hours.
| Tankless model line, application | Heat exchanger | Parts | Labor |
|---|---|---|---|
| HX3 series, single or multi-family | 15 years | 5 years | 1 year |
| HX3 series, commercial | 1 year | 1 year | 1 year |
| 910/T-M50, proper pump control | 15 years | 5 years | 1 year |
| 910/T-M50, without pump control | 3 years | 3 years | 1 year |
| 910/T-M50, commercial or multi-family | 10 years | 5 years | 1 year |
Source: A.O. Smith, Limited Gas Tankless Water Heater Warranty, form 100344051 Rev. A; Limited Warranty (910/T-M50 series), form ATM50, 63V051-6.
Both documents exclude sediment buildup, a closed system with no thermal expansion provision, and installation by anyone other than a licensed contractor; the tankless documents also cap hardness at 7 grains per gallon for a single-family install, or 4 grains otherwise.
AO Smith does not publish repair prices, so these are market-wide averages. Fixr's national data puts the average water heater repair at $450, typically $150 to $750, and the average tankless repair at $375, typically $100 to $650. What changes on an AO Smith unit is whether it is still inside a warranty window.
Past year one, a claim should only cost the diagnostic trip, since the part and labor are covered. After that, the part can still be free inside the tank's or the tankless heat exchanger's warranty window, but labor becomes out-of-pocket, where Fixr's part-level figures apply.
| Repair type | Typical range | National average |
|---|---|---|
| Any-brand tank repair | $150 to $750 | $450 |
| Any-brand tankless repair | $100 to $650 | $375 |
| Heating element, installed | $200 to $750 | not given |
| Thermostat replacement | $100 to $250 | not given |
Source: Fixr, Water Heater Repair Cost, updated 29 August 2025; Fixr, Tankless Water Heater Repair Cost, published 31 January 2025.
The general problems and repair cost page has the full ranges, and the cost guide covers full replacement.
AO Smith's own Find a Service Provider tool, at local.hotwater.com, searches by zip code for an authorized installer or repair company. The company states plainly that everyone listed is an independent contractor, not an employee or agent of AO Smith, so confirming the company's license before it touches gas piping or the electrical connection is still the buyer's job.
For a specific question, AO Smith's own documents list several numbers rather than one general line.
| Need | Contact | Source document |
|---|---|---|
| Customer or technical support | AO Smith Customer Care | Current AO Smith support page |
| Residential technical assistance | AO Smith technical support | FVIR Service Handbook, 2014; tank warranty, 2018 |
| Tankless warranty and support | AO Smith tankless support | Tankless warranty document and 910/T-M50 manual |
| Replacement parts | AO Smith parts fulfillment | AO Smith Water Heater Parts Fulfillment |
The FVIR handbook says it is meant for licensed plumbing professionals, a sign the codes above are best read out to whoever answers one of these numbers rather than resolved at home. Hot Water Brief does not perform repairs, set prices or promise how quickly any company responds.
AO Smith's own guidance puts a tank water heater's average life expectancy at 8 to 12 years and recommends replacement once a unit passes 10 years old and starts leaking or running erratically. Separately, the company states its tankless models typically last 15 to 20 years, roughly double a tank's service life.
Hard water is the factor AO Smith names as the biggest threat to that lifespan, since it drives the annual descaling a tankless unit needs, priced by the company's own site at $200 or more per visit. Its X3 Scale Prevention Technology, built into the HX3 models above, is described as extending operational life by up to 300% by removing that requirement. On the tank side, the anode rod replacement guide covers what checking or replacing that part costs, on any brand.
A. O. Smith's warranty treats the anode as a consumable part, so replacing it is an owner cost; the AO Smith anode rod replacement guide covers the socket size and part numbers.
Every figure on this page traces to an A.O. Smith document or a published cost guide, listed below with a link.
Per AO Smith's FVIR service handbook, a solid status light means the pilot recently went out and the thermopile is cooling. Turn the knob off, wait 10 minutes, then relight.
AO Smith's manual for its 910 and T-M50 series lists code 991 as imperfect combustion, one of more than 20 codes shown on an optional remote control or the unit's own 7-segment LED.
No. AO Smith's residential tank warranty document lists sediment build-up as an event that voids coverage, alongside a system with no provision for thermal expansion.
No. AO Smith's residential tank warranty states anode rods are consumable maintenance parts, excluded from coverage. The anode rod replacement guide covers what checking or replacing one costs.
AO Smith's Find a Service Provider tool searches by zip code for local installers and repair companies, which the company states are independent contractors, not employees or agents.
Independent Research
Published ranges for tank and tankless replacement, the failures that justify a repair, and the ones that do not. Sourced, dated, and free to read.
