A.O. Smith tankless or high-efficiency water heater shows E06 and stops heating

Safety: Turn off power and close gas and water isolation valves before servicing sensors.

• 1. Power cycle: Turn the unit off for 5 minutes, then restore power to clear transient faults.
• 2. Inspect wiring: Locate the outlet temperature sensor (thermistor) on the hot water outlet or heat exchanger. Check that the plug is fully seated on the control board and that wires are not pinched or damaged.
• 3. Check sensor placement: Ensure the sensor is fully inserted into its well or clipped firmly to the pipe as designed.
• 4. Descale if needed: Heavy scale can insulate the sensor. If the unit is tankless or condensing, perform a descaling procedure using a pump and vinegar or approved descaling solution per A.O. Smith instructions.
• 5. Replace sensor: If wiring is intact and E06 persists, replace the outlet temperature sensor with an OEM A.O. Smith thermistor matched to your model.

If replacing the sensor does not resolve the error, the control board may be defective and should be evaluated by a professional.