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Error code reference

A.O. Smith Water Heater Error Code 5

Water temperature in the tank has exceeded 185°F (85°C) and has activated the ECO (Energy Cut Off).

A.O. Smith codesA.O. Smith water heater codes

Water temperature in the tank has exceeded 185°F (85°C) and has activated the ECO (Energy Cut Off).

First step from the service manual

Turn power off for 10 to 20 seconds then on again to clear the error code.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What 5 means on a A.O. Smith water heater

Error Code 5 on an A.O. Smith water heater indicates that the tank water temperature has exceeded 185°F (85°C), triggering the ECO, or Energy Cut Off. The ECO is a thermal safety device designed to interrupt power to the heating elements when dangerously high temperatures are detected. This is a protective shutdown, not a routine operating state, and it signals that something caused the water to overheat well beyond the normal operating range of 120°F to 140°F.

The ECO is typically a one-time-trip or manual-reset component located on the thermostat assembly. When water temperature breaches the 185°F threshold, the ECO opens the circuit to cut power to the heating element, preventing scalding water delivery and potential tank damage. Once tripped, the unit will not resume heating until the fault condition is cleared. A power cycle of 10 to 20 seconds is the first step to attempt a reset, but if the underlying cause is not corrected, the ECO will trip again.

Common root causes include a failed thermostat that is not shutting off the element at the set point, a shorted or stuck heating element, or sediment buildup on the tank floor creating localized hot spots that skew temperature readings. In rare cases, a faulty ECO sensor itself may trip at lower-than-actual temperatures. Repeated tripping of Error Code 5 should be treated seriously, as sustained overtemperature conditions can damage the tank lining, accelerate anode rod depletion, and create scalding hazards at fixtures.

Source: manufacturer service documentation.

Common causes of 5

  1. 01

    Failed or stuck thermostat

    Part replacement

    The thermostat controls when the heating element turns on and off. If it fails in the closed position, the element runs continuously and drives water temperature past 185°F, tripping the ECO. A thermostat that does not cycle the element off at the set point is the most frequent cause of this error.

  2. 02

    Shorted or grounded heating element

    Part replacement

    A heating element that has developed an internal short can draw continuous power regardless of thermostat signals, causing rapid overheating. Testing the element with a multimeter for continuity to ground can confirm this failure.

  3. 03

    Heavy sediment buildup in the tank

    Wiring / connection

    Accumulated sediment on the tank floor can insulate the lower heating element and create localized hot spots, causing temperature readings to be uneven and the ECO to trip. Flushing the tank may reduce this risk, but severe buildup may require element inspection as well.

  4. 04

    Faulty ECO sensor

    Part replacement

    In less common cases, the ECO thermal cutoff device itself may be defective and trip at temperatures below the actual 185°F threshold. If the thermostat and element both test within specification, the ECO component should be evaluated for replacement.

  5. 05

    Thermostat set point too high or adjusted incorrectly

    Wiring / connection

    If the thermostat dial or setting was manually adjusted to an extreme position, it is possible the element is operating at a higher set point than intended, pushing tank temperature toward the ECO trip threshold during recovery cycles.

See the test procedure for each cause

Frequently asked questions about 5

What does 5 mean on a A.O. Smith water_heater?
Error Code 5 means the water temperature inside the tank exceeded 185°F (85°C) and triggered the ECO, which is the Energy Cut Off safety device. The ECO shuts down power to the heating element to prevent scalding and equipment damage. This is a protective fault code, not a routine status, and requires diagnosis to determine why the water overheated.
Will resetting the power clear Error Code 5 on my A.O. Smith water heater?
Turning the power off for 10 to 20 seconds and then back on is the first recommended step and may clear the error code if the temperature has dropped to a safe level. However, if the underlying cause, such as a failed thermostat or shorted heating element, is not corrected, the ECO will trip again and the error will return. A one-time reset that holds is acceptable, but repeated tripping requires a component-level inspection.
Is it safe to use my water heater after Error Code 5 appears?
The ECO has already cut power to protect the system, so the heater is not actively heating when this code is displayed. You should not simply reset and ignore repeated occurrences of this error, as sustained overtemperature conditions can damage the tank lining, stress plumbing connections, and deliver scalding water to fixtures. If the unit trips this code more than once, stop using it for heating and schedule a diagnostic inspection before resuming normal operation.
What does it cost to fix Error Code 5 on an A.O. Smith water heater?
Repair costs depend on the failed component. A thermostat replacement typically runs in the range of $75 to $200 including parts and labor. A heating element replacement is generally in the $100 to $250 range depending on element type and accessibility. If the ECO itself needs replacement as part of a thermostat assembly, costs are similar. A full service call for diagnosis without parts usually runs $75 to $150, which is often applied toward the repair total.
What causes a water heater to overheat and trigger Error Code 5?
The most common causes are a thermostat that has failed in the on position, allowing the heating element to run without interruption, and a shorted heating element that draws continuous power regardless of thermostat control. Sediment buildup inside the tank can also cause localized overheating near the element. Less commonly, a faulty ECO sensor may falsely trip at temperatures below the actual 185°F threshold.

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