A.O. Smith E10 Error Code — What It Means
The E10 error code on an A.O. Smith tankless water heater means the inlet temperature sensor (thermistor) circuit is open. In manufacturer terms, the inlet temperature sensor is not sending a proper signal to the control board. This happens when the sensor’s electrical connection is broken, either by a loose or corroded connector, damaged wiring, or a failed thermistor itself. The control board expects continuous resistance readings from the sensor and throws E10 when that circuit is interrupted.
In most cases, the problem is a loose connector at the inlet thermistor or a faulty sensor. Less commonly, the control board itself can fail to read a good sensor. A.O. Smith’s service documentation points technicians first to the thermistor connection, then to sensor replacement, and finally to the circuit board if the fault persists after those checks.
Common Causes
- Loose or disconnected thermistor connector The plug at the inlet sensor has vibrated loose or was not fully seated during installation or prior service.
- Failed inlet temperature sensor The thermistor itself has an open element and no longer provides a valid resistance reading to the control board.
- Damaged or corroded wiring Wires between the inlet sensor and control board are broken, pinched, or corroded and interrupt the circuit.
- Water intrusion at sensor terminals Moisture has entered the sensor connector or housing and caused corrosion or an open circuit.
- Control board failure The circuit board cannot read the sensor signal even when the sensor and wiring are intact, requiring board replacement.
Step-by-Step Fix
- Turn off power to the water heater at the breaker or disconnect switch to prevent shock during inspection.
- Locate the inlet temperature sensor (thermistor) on the cold-water supply line near where it enters the heat exchanger.
- Inspect and disconnect the sensor connector, checking for corrosion, moisture, or bent pins, then firmly reconnect it.
- Restore power and monitor whether the E10 code reappears during a hot-water draw.
- Replace the inlet thermistor if the code returns immediately or within a few cycles, using the sensor specified for your model.
- Verify wiring continuity between the sensor harness and the control board if a new sensor does not clear the fault.
- Contact A.O. Smith technical support or replace the circuit board if the error persists after confirming the sensor and wiring are good.
Parts Often Needed
| Part | Notes |
|---|---|
| Inlet temperature sensor (thermistor) | Amazon | Primary replacement part for E10. Match the sensor to your exact A.O. Smith model number. |
| Circuit board / control board | Amazon | Required only if E10 persists after sensor and wiring checks. Confirm your model before ordering. |
When to Call a Pro
Call a licensed plumber or technician if you are uncomfortable working with electrical connections on a water heater, if the error returns after reconnecting the sensor, or if you need to replace the thermistor or control board. Tankless water heaters require proper grounding and voltage checks, and misdiagnosis can lead to expensive part replacements. A pro can test sensor resistance, verify board inputs, and confirm that wiring is intact before ordering components.