On an A. O. Smith gas water heater, one flash every three seconds is normal. Two flashes means the thermopile voltage is low, four means the water got too hot, five is a failed temperature sensor, seven a failed gas valve and eight a power-off fault. No light means the pilot is out.
How do I read the A. O. Smith status light?
Watch the light through one full cycle. A. O. Smith says each flash code occurs, then a three second pause, then repeats. Count the flashes before the pause. One flash every three seconds is normal operation, with a bright and dim heartbeat pattern. The light only reports when the pilot is lit and the thermopile, the small power source next to the pilot, has warmed up. A. O. Smith says that can take up to 90 seconds of continuous pilot operation before the light starts to blink.
So if the light is dark while you are holding the knob in, that alone is not a fault. Keep the pilot lit for the full 90 seconds, or until the light starts to blink, before you decide anything.
What does each flash code mean?
This list follows A. O. Smith's service handbook and its installation manual chart.
- No light (0 flashes). The control is off, the pilot is out, or the thermopile has not warmed up yet. Turn the knob to OFF, wait 10 minutes, and relight by the label.
- Solid light. The pilot was just put out and the thermopile is cooling. Turn the knob to OFF, wait 10 minutes, then relight.
- 1 flash every 3 seconds. Normal operation. Nothing to do.
- 2 flashes. The pilot is lit, but the thermopile is not producing the voltage the valve needs. A. O. Smith says the thermopile is probably defective, though loose wiring or a weak pilot flame can cause it. The service handbook says to check the wiring first, then to replace the thermopile if its output is under 350 millivolts DC.
- 4 flashes. The gas valve's temperature sensor saw water that was too hot, so the main burner and pilot shut off. Because the pilot is out, the code appears only right after you relight it. If you see 4 flashes again, turn the knob to OFF and shut off the main gas. The gas control valve then needs to be replaced.
- 5 flashes. The temperature sensor is defective. The fix is to replace the sensor or, in the handbook's wording, the gas control valve.
- 7 flashes. The gas control valve has failed. Replace it.
- 8 flashes. The thermopile kept producing power after the knob was turned OFF. This can happen when the thermopile cools slowly, or when the pilot valve is stuck open. Set the knob to OFF and wait a minute, and check the sight glass. A flame with the knob OFF means the gas control valve must be replaced. With no flame, wait 10 minutes and relight. If it returns, replace the valve.
Two things stand out. The 4, 5, 7 and 8 codes end in gas valve or sensor replacement, which is work for a licensed technician because it involves the gas line. And the first three, no light, solid light and 2 flashes, are about the pilot and the thermopile, which is where most calls start.
How do I relight the pilot to read the code?
Follow the lighting label on your tank, since models differ. A. O. Smith's steps run like this. Turn the knob to OFF and wait ten minutes for any gas to clear. If you then smell gas, stop. Turn the knob to PILOT, press it all the way in and hold it. While holding it in, click the igniter button about once a second for up to 90 seconds or until the status light starts to blink. When it blinks, release the knob and set your temperature.
The valve has a built-in delay. A. O. Smith says its circuitry requires a 10 minute wait between lighting attempts, and that you should stop after three tries. If the status light does not blink after three attempts, turn the knob to OFF and call a qualified service technician or your gas supplier. Repeating the attempts faster will not work, because the valve will not accept them.
A. O. Smith also says that if the pilot flame will not show, the spark igniter or the gas supply to the pilot should be checked. On a new install or after gas work, air in the line may take several tries to clear.
What about a gas smell or a discolored flame arrestor?
If you smell gas, do not touch the igniter or any switch. Leave, then call from outside. The standard warning on gas heaters is to call your gas supplier, and in the Phoenix area Southwest Gas asks you to call 911 and 877-860-6020 from a safe place and to leave the doors open on your way out. Our page on what to do when the pilot is out and you smell gas covers why a relight has to wait.
There is one more safety point on these heaters. The FVIR design has a flame arrestor and a thermal switch. A. O. Smith's handbook says that if a tripped switch resets with a click, you should check the flame arrestor for discoloration, which can be caused by flammable vapors. If the arrestor is discolored, do not try to relight the pilot. Have a qualified technician inspect the heater. Keep gasoline, solvents and aerosols out of the room, and keep the air intake at the base free of lint and dust.
### Which chart applies to my tank?
The codes above come from A. O. Smith's manuals for its residential gas models with a Honeywell style valve, including the GCV and GVR 300 series in the service handbook and the direct vent models in the installation manual. Other series and older tanks can use different counts. Your rating plate lists the model, and A. O. Smith's manuals point to hotwater.com for the exact manual. Match your model before you act on a count. The manual for your tank rules over any chart, including this page.
For a heater that shuts itself off without a clear flash code, see why a gas water heater keeps shutting off, which walks through the thermal switch and the draft test.
When should I call a plumber?
Call when you see 4, 5, 7 or 8 flashes, when the pilot will not stay lit after three attempts spaced 10 minutes apart, when the thermopile output tests low, or when the flame arrestor looks discolored. Those are the points where A. O. Smith's own charts say to replace a part or stop.
For a stubborn flash code, HQ Plumbing and Air sends licensed, insured technicians at any hour. The estimate is free and the price is quoted before the work begins.
Related reading: if the pilot goes out when you release the knob, see our page on why a water heater pilot light will not stay lit. If you own a Rheem, see our page on Rheem water heater blinking light codes, which use a similar gas valve with slightly different counts. And because Phoenix water is hard, our page on how often to flush a water heater in Phoenix helps keep sediment from shortening the life of any brand of tank.
For another brand with a similar light, see our page on Bradford White flashing light codes.