State Water Heater E51 Error Code — What It Means
The E51 error code on State tankless water heaters signals a temperature sensor (thermistor) problem. The control board has detected that one of the thermistors is reading abnormal resistance or has lost communication with the sensor. This code appears in A. O. Smith and State-family tankless models and requires a diagnostic check of sensor resistance and wiring before any parts are replaced.
The heater will not operate normally until the faulty sensor or control board is addressed. Because State does not publish a standalone E51 definition on its public support pages, this guidance is drawn from A. O. Smith tankless service manuals that share the same fault-code architecture.
Common Causes
- Failed thermistor The temperature sensor itself has degraded or shorted, causing resistance readings outside the normal range.
- Loose or corroded sensor connector Poor contact at the thermistor plug can mimic a sensor failure and trigger the E51 code.
- Damaged sensor wiring Chafed, pinched, or broken wires between the thermistor and control board interrupt the signal.
- Control board failure If all sensors test within spec, the circuit board’s sensor-input circuitry may be faulty.
Step-by-Step Fix
- Turn off electrical power at the breaker and close the gas shutoff valve to the water heater before starting any work.
- Run hot water at a nearby fixture for about five minutes to flush the heat exchanger and allow the unit to reset to a fresh diagnostic state.
- Remove the front cover and locate the thermistor connectors on the control board or heat-exchanger manifold.
- Measure thermistor resistance using a digital multimeter set to ohms, comparing the reading of each sensor; if one sensor’s resistance is significantly higher than the others, that thermistor has failed.
- Disconnect and reconnect each thermistor plug firmly to rule out a poor connection, then restore power and check if the code clears.
- Replace the faulty thermistor if resistance is abnormal or the sensor connector shows corrosion; use the exact replacement part for your model.
- Replace the control board if all sensors test good, connectors are clean, and the E51 error persists after sensor checks.
Parts Often Needed
| Part | Notes |
|---|---|
| Temperature sensor (thermistor) | Amazon | Order by your State tankless model number; resistance spec varies by heater family. |
| Control board (circuit board) | Amazon | Required only if thermistor tests pass but the E51 fault remains. |
When to Call a Pro
Call a licensed plumber or gas technician if you are uncomfortable working with 120 V electrical connections or natural-gas appliances. If you lack a multimeter or are unsure how to interpret thermistor resistance readings, professional diagnosis will save time and prevent incorrect part orders. Any time the error persists after you have replaced the thermistor and verified all connectors, the control board is the likely culprit and a technician can confirm the board fault before you invest in that more expensive component.