State Tankless Water Heater E02 Error — What It Means
The E02 error on a State tankless water heater signals that the control board has detected outlet water temperature above the allowed limit and has shut down the unit as a safety precaution. The fault is tied directly to the temperature sensor circuit. When the thermistor reads abnormally high or the heated water actually exceeds the threshold, the heater locks out and displays E02to prevent scalding or equipment damage.
In State and A.O. Smith service documentation this code points first to the thermistor itself. If the sensor gives an incorrect reading or its wiring is loose or corroded, the board interprets the signal as dangerously hot water and triggers the fault even when actual outlet temperature is normal.
Common Causes
- Faulty thermistor or temperature sensor The sensor reads incorrectly high and tells the board the water is too hot when it is not.
- Loose, corroded, or damaged sensor wiring Intermittent contact or corrosion at the thermistor connector causes erratic high-temperature signals.
- Temperature setpoint adjusted too high A setpoint at or near the unit’s maximum can push outlet temperature over the safety limit during normal operation.
- Control board fault The board misreads a good sensor signal or fails to regulate heating correctly, triggering a false over-temperature lockout.
- Incoming water below 40°F State service material notes that very cold inlet water can contribute to fault conditions and must be corrected before restoring service.
Step-by-Step Fix
- Turn off power at the breaker and wait 30 seconds, then restore power to reset the control board and clear the E02 code.
- Lower the temperature setpoint by 5–10°F and run hot water at a fixture for two minutes to see if the heater operates without faulting.
- Inspect the thermistor wiring at the sensor and control-board connectors for loose pins, corrosion, or physical damage, and reseat every connector firmly.
- Measure thermistor resistance at the sensor with a multimeter and compare the reading to any other temperature sensors on the unit. Replace the thermistor if one sensor’s resistance is much higher than the others or if the reading is erratic.
- Replace the thermistor if it tests abnormal or if cleaning the connector does not resolve intermittent faults.
- Replace the control board if the thermistor and wiring test normal, the setpoint is reasonable, and the E02 error returns immediately on every heating cycle.
- Verify inlet water temperature is above 40°F if the heater is installed in an unheated space or supplied by very cold well water. Insulate the cold-supply line or wait for warmer conditions before returning the unit to service.
Parts Often Needed
| Part | Notes |
|---|---|
| Thermistor / temperature sensor | Amazon | Order the sensor assembly by your model number. Resistance values are model-specific. |
| Temperature sensor wiring harness | Amazon | Replace if the connector or wire insulation is burned, cracked, or corroded. |
| Control board / circuit board | Amazon | Required when sensors test good but E02 persists. Match the board to your exact model and serial number. |
When to Call a Pro
Call a licensed plumber or water-heater technician if you are uncomfortable working with 120 or 240 V electrical connections, if you cannot locate or access the thermistor, or if the fault returns after you have reset the unit and verified wiring. A technician has the meter, resistance tables, and experience to compare thermistor readings across sensors and to replace the control board safely. Professional help is also the right choice if your unit is under warranty, because self-repair may void coverage.