6 Flashes
AO Smith Water Heater
Severity: CriticalWhat Does This Error Mean?
AO Smith status LED flashing 6 times means the tank temperature exceeded the safety threshold — typically 180-199°F depending on model.
The gas valve closed and locked out.
This is rare during normal operation: a working thermostat keeps water well below this point.
Cause is usually a stuck gas valve, a failed sensor, or a thermostat sticking on after years of mineral fouling.
Affected Models
- AO Smith ProMax
- AO Smith Signature Premier
- AO Smith Vertex (gas backup)
- AO Smith FCG series
Common Causes
- Gas valve mechanically stuck open
- Tank temperature sensor reading low and overheating the actual water
- Thermostat dial set above 140°F combined with a long no-draw period
- Heavy scale on the sensor probe insulating it from real water temperature
- Recent service work where the sensor was reinstalled incorrectly
How to Fix It
-
Stop using hot water.
Water in the tank may be scalding hot.
Do not run a tap until the tank cools.
Test gingerly at the closest tap before letting anyone bathe or wash hands. -
Power down and gas off.
Turn the gas valve to OFF.
If the unit has a dedicated electrical circuit (powered models), switch the breaker off.
Let the tank cool for at least an hour before further work. -
Drain a small amount and check.
Open a hot tap furthest from the tank.
Let it run carefully into a sink — let the tap run cool first.
Confirm that hot water comes out at a reasonable temperature once the system is back below 140°F. -
Inspect the sensor probe.
Remove the access cover near the gas valve.
The sensor is a small probe held against the tank by a clip.
Heavy white scale on the probe tip means the sensor cannot read the water — it reads a low temperature, the gas valve keeps firing, and the tank overheats. -
Clean or replace the sensor.
Wipe the probe and the sensor pocket with descaling solution and a soft brush.
If the probe surface is pitted or the cable is brittle, replace the sensor (15-50 USD). -
Test the gas valve.
If the sensor is fine and 6 flashes returns after a fresh fill, the gas valve is sticking mechanically.
Replacement gas valves are 150-350 USD.
This is a job for a plumber — mismatched gas valves cause CO production. -
Lower the thermostat.
Once back online, set the thermostat no higher than 120°F for daily use.
Higher settings push the unit closer to the over-temperature threshold and increase scale buildup.
Frequently Asked Questions
Why is a 140°F setting risky?
Above 130°F the scald risk to children and elderly users rises sharply.
Above 140°F the over-temperature threshold is much closer to normal operating point.
The Plumbing Code in many areas now mandates 120°F default for these reasons.