A.O. Smith E2 Error Code — What It Means
E2 (sometimes displayed as E02) is an over-temperature safety fault on A.O. Smith electric tankless water heaters. The unit has detected water temperature at or above the safe operating limit and has shut down to protect the heater and your plumbing. The control board monitors water temperature through thermistor sensors, and this code fires when the sensor circuit reports dangerously hot water or a sensor reading that appears to indicate overheating.
Common Causes
- Failed or drifting thermistor sensor The temperature sensor can fail open, short, drift out of calibration, or send incorrect resistance readings to the control board.
- Loose, corroded, or damaged thermistor wiring Poor connections, heat damage, or corrosion at the sensor connector or harness can create intermittent or false high-temperature signals.
- Actual overheating from low flow or high setpoint Insufficient water flow through the heater or a setpoint that is too high can allow the water to climb above the safety threshold and trip the protection.
- Control board fault If the thermistor and wiring test normally but the E2 code keeps returning, the control board itself may be misreading or mishandling the temperature signal.
Step-by-Step Fix
- Turn off power at all circuit breakers feeding the water heater before opening any panels or touching components.
- Run a hot-water fixture for a few minutes to flush the heater and clear any trapped high-temperature water, then close the tap and let the unit sit for a moment.
- Inspect the thermistor wiring and connectors for loose pins, heat damage, melting, or corrosion at both the sensor and the control board.
- Measure the resistance of each thermistor using a multimeter and compare the readings against each other and your model’s specification table (found in the installation manual or service documentation).
- Replace any thermistor that reads much higher resistance than the others or falls outside the manufacturer’s expected range for the current water temperature.
- If the sensor and wiring check out but the E2 persists, replace the control board or contact A.O. Smith technical support for model-specific guidance.
- Restore power and test by running hot water at normal flow to verify the fault clears and the heater operates without tripping E2 again.
Parts Often Needed
| Part | Notes |
|---|---|
| Thermistor / temperature sensor | Amazon | Primary suspect when resistance is abnormal or out of range. Order the sensor specified for your exact A.O. Smith model. |
| Thermistor wiring harness | Amazon | Replace if connectors are melted, corroded, or the wiring shows heat damage or breaks. |
| Control board / circuit board | Amazon | Required if the thermistor and wiring test good but the E2 fault returns. Confirm part number before ordering. |
When to Call a Pro
Call a licensed technician if you are uncomfortable working inside energized electrical appliances, if you cannot safely measure thermistor resistance or interpret the readings, or if replacing the sensor does not clear the E2 code. A pro has the manufacturer resistance tables, diagnostic tools, and access to A.O. Smith technical support to pinpoint control-board faults or intermittent wiring issues that are hard to catch with a single test. If your heater is still under warranty, professional service may be required to preserve coverage.