2 Flashes
AO Smith Water Heater
Severity: ModerateWhat Does This Error Mean?
AO Smith status LED flashing twice means the thermopile is generating less voltage than the gas control needs to stay on.
The pilot lights, the LED flashes 2, and then the main burner refuses to fire.
Cause is almost always a dirty thermopile, a weak pilot flame not fully wrapping the probe, or a thermopile that has aged out and needs replacement.
Affected Models
- AO Smith ProMax
- AO Smith Signature Premier
- AO Smith Promax Plus
- AO Smith Vertex (gas backup mode)
- AO Smith FCG series
Common Causes
- Thermopile coated with soot or scale
- Pilot flame too small or yellow instead of blue
- Pilot tube partially blocked
- Thermopile aged out — typical life is 4-7 years
- Loose connection at the gas valve thermopile terminal
How to Fix It
-
Power down, gas off.
Turn the gas valve to OFF.
Wait 5 minutes for the pilot area to cool before working.
Gas valves carry millivolt signals — test gear can use the same terminals safely later. -
Inspect the pilot flame.
Restart the pilot per the lighting instructions on the unit.
The flame should be blue and fully wrap the upper third of the thermopile probe.
Yellow flame, small flame, or a flame that does not touch the probe means a pilot tube issue, not a thermopile failure. -
Clean the thermopile.
Shut the gas, let everything cool.
Remove the burner assembly access panel.
Wipe the thermopile probe with fine emery cloth.
Reinstall and re-light. -
Test thermopile voltage.
With a millivolt meter on the gas valve thermopile terminals, light the pilot and let it stabilise.
Healthy thermopiles read 650-850 mV.
Below 400 mV under load means a failed thermopile. -
Replace the thermopile.
Genuine AO Smith thermopile assemblies are 25-60 USD.
Disconnect at the gas valve, pull the assembly out, slide the new one in.
Tighten finger-tight at the gas valve plus a quarter turn with a wrench — over-tightening damages the terminal. -
Clear the pilot tube.
If the pilot flame stays small after a new thermopile, the pilot orifice or tube is partially blocked.
Compressed air through the pilot supply line, or a small vacuum at the burner end, dislodges most debris.
Frequently Asked Questions
Can I just clean instead of replacing?
Cleaning works on a thermopile that has been recently dirtied.
If the unit is 5+ years old and cleaning gives only a brief reprieve, the thermopile has reached end of life.
Cheaper to swap once than to clean every month.