How do I fix A.O. Smith Water Heater High-Limit Switch/ECO Replacement?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with this fault active?

Generally no. Faults on Water heater units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for A.O. Smith Water Heater High-Limit Switch/ECO Replacement?

The most frequently replaced parts for A.O. Smith Water Heater High-Limit Switch/ECO Replacement are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is A.O. Smith Water Heater High-Limit Switch/ECO Replacement a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

A.O. Smith Water Heater High-Limit Switch/ECO Replacement

Q: What does A.O. Smith Water Heater High-Limit Switch/ECO Replacement mean?

No hot water or tripped reset button means the high-limit switch has shut off power. Replacing the upper thermostat/ECO assembly restores safe heating.

A.O. Smith Water Heater High-Limit Switch/ECO Replacement — What This Part Does

The high-limit switch (ECO or Energy Cut Off) is built into the upper thermostat assembly on A.O. Smith electric water heaters. It monitors tank temperature and opens the circuit to cut all power to the heating elements if the upper tank area overheats, preventing scalding water or tank damage. This is a one-time safety trip that requires a manual reset.

The ECO trips because the upper tank section has actually overheated, or the upper thermostat has failed and is no longer controlling temperature correctly. A failed or weak upper heating element can also cause abnormal heating cycles that trip the switch. If the reset button trips repeatedly after being pressed, the root cause is a failed element, failed thermostat assembly, or loose wiring at the upper terminal block.

Jump to Replacement Steps

Signs It Needs Replacing

No hot water at all The water heater produces no hot water and the reset button on the upper thermostat has popped out or feels loose.
Reset button trips again immediately You press the small red reset button under the upper access panel but it trips again within minutes or hours.
Water was scalding hot before shutdown The water temperature became dangerously hot just before the heater stopped heating completely.
Breaker is on but no heating The circuit breaker at the panel is in the on position but the water heater will not heat.
Burnt smell or discolored wiring at upper thermostat You see melted insulation, blackened terminals, or smell burnt plastic near the upper thermostat area.
Clicking sound when you press reset The reset button clicks when pressed but the heater still does not heat after power is restored.

How to Replace It

Turn off the circuit breaker dedicated to the water heater at your electrical panel.
Shut off the cold water supply valve at the top of the tank.
Remove the upper access panel screws and lift off the metal cover.
Pull away the insulation pad to expose the upper thermostat and wiring.
Take a photo of all wire connections on the upper thermostat before disconnecting anything.
Use a non-contact voltage tester to confirm no power is present at the top two terminal screws of the upper thermostat.
Disconnect all wires from the upper thermostat by loosening the terminal screws (typically two power feed wires at the top, two wires to the upper element, and two or more wires running down to the lower thermostat).
Lift the old upper thermostat out of its mounting bracket or clips.
Snap or slide the new upper thermostat with integrated ECO into the mounting bracket against the tank wall.
Reconnect all wires to the new thermostat in the same positions as the original, matching your reference photo.
Set the temperature dial on the new upper thermostat to match the lower thermostat setting (typically 120°F).
Press the ECO reset button on the new thermostat until it clicks and seats flush.
Replace the insulation pad and upper access panel, then turn on the cold water supply and check for leaks at the tank fittings.
Turn the circuit breaker back on and listen for the upper element to begin heating (a faint hum or click from the thermostat relay).
Wait 1-2 hours and check for hot water at a faucet, then verify the reset button has not tripped again.

The Part You Need

Part	Notes
Upper thermostat with high-limit switch (ECO)	Amazon \| Match the model and serial number on the data plate on the side of your water heater tank. A.O. Smith thermostats are model-specific for voltage and element wattage.
Upper heating element	Amazon \| Replace if resistance measures outside 5-25 ohms (12.5 ohms typical). Match wattage and voltage on the old element or the tank data plate.

If this part is failing you may also see one of these codes:

When to Call a Pro

Call a licensed electrician or water heater technician if you are not comfortable working with 240-volt wiring or if you do not get a clear 240 V reading at the upper thermostat when the breaker is on. If the new thermostat and element still cause the ECO to trip, there may be a short in the tank wiring harness, a grounding fault inside the tank jacket, or a problem with the home electrical supply that requires professional diagnosis. Repeated ECO trips after parts replacement can also mean the tank itself has sediment buildup or internal corrosion that is causing localized overheating, and a pro can pressure-test and inspect the tank for replacement. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.