A.O. Smith Tankless E06 Error — What It Means
The E06 code on an A.O. Smith electric tankless water heater indicates that the control has detected an overtemperature condition. The water inside the unit is hotter than the safe operating threshold, and the heater may shut down to protect itself. This fault can appear on power-up if the unit already senses excessive temperature, or it can trip during operation if conditions allow internal temperature to climb beyond normal limits.
Common Causes
- Insufficient water flow or no flow through the heater When water stops moving or circulates too slowly, the heating elements raise the temperature of the standing water inside the unit above safe limits.
- Thermistor connection issue A loose, corroded, or intermittent connection at the T-in thermistor or its wiring can cause the control to read false high-temperature signals.
- Failed T-in thermistor If the thermistor’s resistance is much higher than the others, it sends incorrect temperature data to the board and triggers an E06 fault.
- Actual overheating event Abnormal operating conditions like extremely high inlet temperature, closed isolation valves, or a stuck heating element can drive real water temperature above the threshold.
Step-by-Step Fix
- Open a nearby hot-water fixture and let water run through the heater for about five minutes to restore flow and allow cooler water to circulate through the unit.
- Watch the display while water flows. If the E06 code clears and does not return, the issue was likely stagnant water or temporary insufficient flow rather than a failed part.
- Turn off power to the heater at the breaker if the code remains after restoring flow for five minutes.
- Inspect the T-in thermistor connections at the sensor and at the board. Disconnect and reconnect each terminal to eliminate a poor contact.
- Measure the resistance of each thermistor if you have an ohmmeter. Replace any thermistor that reads much higher than the others on the same unit.
- Restore power and test with a hot-water tap open. If E06 reappears immediately, the thermistor or board may need replacement.
- Contact A.O. Smith Technical Support if the fault persists after flow restoration and thermistor checks, as the manual directs escalation at that point.
Parts Often Needed
| Part | Notes |
|---|---|
| T-in thermistor | Amazon | Match the part number stamped on your existing sensor or printed in your model’s service manual. |
| Thermistor wiring harness | Amazon | Order from A.O. Smith if connector terminals are corroded or damaged beyond cleaning. |
When to Call a Pro
If the E06 code does not clear after you run water through the heater for five minutes, or if it clears but returns every time you power the unit on, you should call a licensed technician or contact A.O. Smith Technical Support. Thermistor testing requires a multimeter and familiarity with resistance checks, and replacing sensors or investigating board-level faults is outside typical DIY scope. Because the fault can also indicate a real overheating condition caused by closed valves or other system issues, a professional can trace flow paths and test heating-element operation safely.