Electric AO Smith water heaters have upper and lower thermostats controlling the upper and lower elements. Failure pattern tells you which one.
Symptoms by failure
- Upper thermostat failed: no hot water at all — upper is primary control
- Lower thermostat failed: hot briefly then quickly lukewarm
- High-limit reset tripped: red button on upper thermostat — reset once. Trips again = thermostat failure or runaway-heating fault
AO Smith thermostat part numbers
- Upper (with reset): 9000-666170 (volume residential)
- Lower: 9000-666175
- Voltex specialized: 9000-666180 (HP control integration)
Replacement procedure
- Shut off power, verify
- Remove access panel for the thermostat
- Pull aside insulation
- Photograph wiring before disconnecting
- Disconnect 4 wires (2 line-in, 2 element-out)
- Release thermostat retaining clip with flathead screwdriver
- Pull thermostat out — sits against tank wall
- Install new thermostat, reattach clip — firm tank-wall contact is critical
- Reconnect wires per photo
- Reset high-limit on upper thermostat
- Set temperature (factory default 120°F)
- Restore power, wait 60-90 min, test
Common mistakes
- Loose tank-wall contact — thermostat reads cooler than actual, over-fires
- Reversed wiring — short circuit
- Forgetting to reset high-limit
Replace both at once?
If unit is past year 8 and one thermostat failed, consider replacing both — second one statistically follows within 1–2 years.
Bottom line
Thermostat is $30-$45 part and 45-min DIY. For other parts see AO Smith parts hub.