Safety first: Disconnect power and allow the unit to cool if it has overheated.

Steps:

Verify actual water temperature: Draw hot water and measure with a reliable thermometer. Compare with the displayed temperature.
Check NTC sensors: Measure resistance of the main and outlet NTC sensors at a known temperature (e.g., room temperature). Compare to manufacturer’s resistance table if available. Replace any sensor with abnormal readings.
Factory reset (if supported): Some Tesy models allow a factory reset via button combination. Perform the reset according to the user manual to clear calibration errors.
Firmware update: If the model supports service firmware updates (via service port), have an authorized service center reflash the firmware.
Replace control board: If sensors are good and reset does not help, replace the electronic control board to restore proper temperature regulation.

Warning: Do not continue using the heater if it overheats water beyond safe limits; scalding risk is significant.

Tesy heater shows E09; water does not heat to set temperature or overshoots significantly.

Possible Causes