Quick answer from a licensed plumber: on an A. O. Smith gas tank water heater with a status light on the gas control valve, one flash every three seconds is normal: the pilot is lit and the valve sees no problem. No light means the pilot is out or the thermopile has not warmed up yet. A solid light means the pilot just went out; wait 10 minutes before relighting. 2, 4, 5, 7 and 8 flashes are faults that can end in a new thermopile, temperature sensor or gas control valve, and A. O. Smith says servicing the heater shall be done by qualified service personnel only. Relighting from the label and cleaning the air filter around the base are yours to do. These meanings come from A. O. Smith’s direct-vent manual; check the chart in your own heater’s manual.
Safety first: if you smell gas, do not try to light any appliance, do not touch any electrical switch or use any phone in the building, and call your gas supplier from a neighbor’s phone (the fire department if you cannot reach them). That is A. O. Smith’s own instruction. A. O. Smith adds: use only your hand on the gas control knob — if it will not push in or turn by hand, do not force it or try to repair it; call a qualified service technician. And if any part of the heater has been under water, it must be replaced, not repaired.
Status Light Codes at a Glance
This is A. O. Smith’s Status Light and Diagnostic Code Troubleshooting Chart from its Use & Care Guide for direct-vent residential gas models (100341469). A. O. Smith’s atmospheric-vent guide (100263112) describes the same normal signal: one flash every three seconds when the pilot is lit and the valve has detected no problem. If your heater has a power cord and a motorized flue damper on top, it uses a different control; skip to that table further down.
| Status light | What A. O. Smith says it means | What to do |
|---|---|---|
| No light (0 flashes) | The pilot is not lit, or the thermopile has not yet reached operating temperature. | Knob to OFF, wait 10 minutes, relight from the label. It can take up to 90 seconds of pilot before the light starts to blink. |
| Solid red | The pilot went out recently and the thermopile is cooling down. | Knob to OFF, wait 10 minutes, then relight. |
| 1 flash every 3 seconds | Normal operation. | Nothing. If there is still no hot water, A. O. Smith says to make sure the knob is set to HOT. |
| 2 flashes | The pilot is lit, but the thermopile is not producing the required voltage. | Probably a defective thermopile; loose wiring or a weak pilot can do it too. Professional. |
| 4 flashes | The valve’s sensor saw the water get too hot; the burner and pilot shut off. Shows only right after a relight. | Knob and main gas OFF; the gas control valve gets replaced. Professional. |
| 5 flashes | The temperature sensor (thermistor) is defective. | Knob OFF; the sensor gets replaced. Professional. |
| 7 flashes | Gas control valve failure. | Knob and main gas OFF; the valve gets replaced. Professional. |
| 8 flashes | The thermopile kept producing power after the knob was turned off — slow to cool, or the pilot valve is stuck open. | Knob OFF, wait a minute, look for a pilot flame (see below). |
Relighting the Pilot the Way A. O. Smith Says
No light and a solid light both end in a relight. The pilot is lit by the piezo igniter; A. O. Smith says not to open the inner door and try to light it by hand. The steps from A. O. Smith’s lighting label (read the one on your heater first):
- Turn the gas control knob counterclockwise to OFF and wait 10 minutes to clear out any gas. If you then smell gas, stop and follow the gas-smell steps above.
- Turn the knob clockwise to PILOT.
- Push the knob all the way in (it travels about 1/4 inch) and hold it. While holding it, click the igniter button about once a second for up to 90 seconds, or until the status light starts to blink.
- When the light blinks, release the knob and set it to the temperature you want.
- If the light does not start blinking within 90 seconds, repeat, up to three times, waiting 10 minutes between attempts. If the light turns solid, release the knob, wait 10 minutes and try again.
- If the light still does not blink after three attempts, turn the knob to OFF and call a qualified service technician or your gas supplier.
A. O. Smith’s “No Hot Water” steps add a simple check before that last call: dim the room lights and watch through the viewport while you click the igniter. No spark? Make sure the igniter wire is firmly seated in its connector. A spark but no pilot? Make sure the gas supply valve to the heater is on.
The Reset Button: Stop and Think Before You Press It
If the light still will not blink after three tries, A. O. Smith’s chart says to check that the heater is getting gas, then remove the outer door and press the reset button. That button belongs to the thermal switch. A. O. Smith’s troubleshooting guide says that if the thermal switch tripped, it is likely an air supply problem: clean the air filter and the flame arrestor, and make sure the room supplies enough air. Its Use & Care guide for the damper models goes further: the switch shuts the gas off when there is poor combustion caused by a blocked vent or insufficient combustion air, and those problems must be corrected before the switch is reset — a venting or combustion-air problem is a job for a qualified technician, not an owner.
Our advice: a thermal switch that trips a second time is a venting or combustion-air problem until proven otherwise, and that can mean carbon monoxide. A. O. Smith says a heater that fails its draft test must not be run until the vent system and air supply have been checked by a qualified person. Shut the gas off and call one.
I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.
The Fault Codes, One by One
2 flashes: weak thermopile signal
The pilot is lit, but the thermopile is not making enough voltage. A. O. Smith says the thermopile is probably defective, though loose wiring or a weak pilot flame can cause the same code. A. O. Smith also notes that a dirty air filter can let the pilot go out, so clean it (below). A. O. Smith’s replacement steps fit the new pilot/thermopile assembly with the manifold and burner assembly removed from the combustion chamber — qualified service work.
4 flashes: the water got too hot
The valve’s built-in temperature sensor saw the water go over its limit and shut down the burner and the pilot. Because the pilot is off, A. O. Smith says you only see this code right after a relight. Its instruction is to relight, confirm the 4 flashes, turn the knob and the main gas supply off, and replace the gas control valve. Treat the water in the tank as scalding until then. If the temperature and pressure relief valve is discharging large amounts of very hot water, A. O. Smith says to turn the gas off and call a qualified person, and never to cap or plug that valve or its discharge pipe.
5 flashes: temperature sensor
A. O. Smith says the thermistor is defective and needs replacing. Knob off and call for service.
7 flashes: gas control valve failure
Knob off, main gas supply off, and the gas control valve gets replaced. A. O. Smith’s replacement steps include draining the tank and end with leak-testing the gas connections — licensed work.
8 flashes: power after shutdown
This only shows after the knob has been turned off. Either the thermopile is cooling slower than expected, or the pilot valve is stuck open and the pilot is still burning. A. O. Smith’s check: with the knob on OFF, wait one minute, remove the outer door and look through the sight glass. If you see a pilot flame, turn off the main gas supply — the valve must be replaced. If there is no flame, wait 10 minutes and relight; if the code comes back, the valve must be replaced.
If Your Heater Has a Power Cord and a Flue Damper
A. O. Smith’s plug-in models with a motorized flue damper (Use & Care Guide 100278381) use a different control, and the patterns do not mean the same thing. The biggest trap: on these models, seven flashes is a flammable-vapor lockout, not a gas valve failure.
| Status light | What A. O. Smith says it means |
|---|---|
| Short flash every 4 seconds | Idle: no call for heat, no fault. |
| “Heartbeat,” alternating bright and dim | Heating, no fault. |
| 1 flash, 3-second pause | Low flame signal (the control keeps running). |
| 2 flashes | End switch failed closed (flue damper). |
| 3 flashes | End switch failed open, or TCO limit lockout. |
| 4 flashes | ECO limit lockout (water too hot). |
| 5 flashes | Flame out of sequence. |
| 6-1, 6-2, 6-3, 6-4 | Soft lockouts: failed ignition retries, flame lost, flame out of sequence. |
| 7 flashes | Flammable vapor sensor lockout. |
| 8-1, 8-2, 8-3, 8-4 | Flammable vapor sensor fault, temperature sensor fault, electronic fault, gas control valve fault. |
For the 8-series faults and a 4-flash ECO lockout, A. O. Smith’s first step is to switch the control’s power switch off for 10 to 20 seconds and back on; if the code persists, the gas control valve gets replaced. For a 7-flash flammable-vapor lockout, it says: do not touch any electrical switch, do not use a phone in the building, do not light any appliance, smell for gas, remove anything that gives off flammable vapors — gasoline, paint, thinners, varnish, cleaners — and contact a certified service representative. The same manual says only a qualified service person can reset the heater after the vapor sensor trips.
Maintenance That Prevents the Codes
- Air filter around the base. A. O. Smith says to inspect it at least once a year, vacuum off dust and debris, and wash it in soap and water if there is an oily residue. A clogged filter can starve the pilot.
- Flame arrestor (flame trap). If debris collects on it, A. O. Smith says to remove it with a vacuum, compressed air or a soft-bristle brush; its figure shows checking from underneath with a mirror and flashlight.
- Anode rod. Inspect it after three years at most, then every year; every year from the start if your water is softened. See water heater anode rod replacement.
- Burner flame. Look through the viewport: A. O. Smith says a soft blue flame is correct, blue tips with yellow inner cones are satisfactory and the tips may have a slight yellow tint, but the flame should not be all yellow. If it is not right, A. O. Smith says to contact a qualified service technician.
When to Stop and Call a Licensed Professional
- You smell gas, or you see a pilot flame with the knob on OFF (8 flashes).
- The thermal switch trips again after you reset it, or you smell exhaust near the heater.
- Any 2, 4, 5 or 7 flash code on the thermopile valve, an 8-flash code that returns after a relight, or a 7-flash vapor lockout on a damper model.
- The relief valve discharges large amounts of very hot water.
- The pilot will not stay lit after three proper attempts.
- The heater has been under water — A. O. Smith says replace it, do not repair it.
Older standing-pilot heaters without a status light work differently; see water heater pilot light won’t stay lit.
FAQ
How often should the status light blink on an A. O. Smith water heater?
On the thermopile-powered gas valves covered here, one flash every three seconds means normal operation: the pilot is lit and the valve has not detected a problem. The chart in the manual pocket on your heater is the final word for your model.
Why is the status light on my A. O. Smith solid instead of blinking?
A solid light means the pilot was lit and has gone out, and the thermopile is cooling down. A. O. Smith says to turn the knob to OFF, wait 10 minutes and relight following the label, because the valve’s circuitry requires the 10-minute wait between attempts.
How many times should I try to relight the pilot?
A. O. Smith’s label says up to three attempts, waiting 10 minutes between them. If the status light still does not start blinking after three tries, turn the knob to OFF and call a qualified service technician or your gas supplier.
What do 2 flashes mean on an A. O. Smith gas water heater?
The pilot is lit but the thermopile is not producing enough voltage. A. O. Smith says the thermopile is probably defective, though loose wiring connections or a weak pilot flame can cause the same code. A. O. Smith’s steps fit the new pilot/thermopile assembly with the manifold and burner assembly removed, and it says servicing the heater shall be done by qualified service personnel only.
Can I replace the gas control valve myself?
A. O. Smith’s manual says servicing the water heater shall be done by qualified service personnel only. A. O. Smith’s steps for swapping the gas control valve include draining the tank, breaking and remaking gas connections and leak-testing them, which is licensed gas work.
Why Trust Plumbing Picks
Journeyman Plumber — State of Georgia Licence #JP507711
Plumbing Picks is written by a Georgia journeyman plumber, and the license can be checked through the Georgia Secretary of State’s license verification portal. Every code meaning and step on this page comes from the manufacturer documents listed below, which we opened and read; where we add our own advice, the page says so. This page is general information for owners, not an offer of repair work. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- A. O. Smith: Residential Gas Water Heaters, Installation Instructions and Use & Care Guide, direct-vented gas models (100341469, March 2021) — Lighting Instructions (p. 27), Status Light and Diagnostic Code Troubleshooting Chart (pp. 43–44)
- A. O. Smith: Residential Gas Water Heater Installation Instructions and Use & Care Guide (100263112, January 2015) — Lighting, No Hot Water, T&P and air filter sections
- A. O. Smith: Maintenance & Troubleshooting Guide — Electronic Residential Gas Valve Troubleshooting (pp. 15–16)
- A. O. Smith: Residential Gas Water Heater with FVIR, Installation Instructions and Use & Care Guide (100278381, October 2016) — Status Light Code table and corrective actions (pp. 21–22, 30–31)
Bottom Line
Count the flashes and match them to the chart in your heater’s manual pocket. One flash every three seconds is normal; no light or a solid light means relight it properly, with 10-minute waits. Clean the air filter and the flame arrestor. Anything past that — 2, 4, 5, 7 or 8 flashes, a thermal switch that keeps tripping, or a flammable-vapor lockout — is work A. O. Smith assigns to qualified service personnel.