Water temperature is excessively hot; risk of scalding at taps.

Safety first: Immediately turn off power to the heater. Warn occupants about scalding risk and, if possible, mix with cold water before use.

Steps:

• Check thermostat: On mechanical models, remove the thermostat and test continuity while turning the knob. If it remains closed regardless of setting, replace the Tesy mechanical thermostat.
• Verify sensor placement: On digital models, ensure the NTC sensor is firmly in contact with the tank or fully inserted into its well. A loose sensor can cause overheating.
• Inspect control relay: If the element remains energized when the set temperature is reached, the relay on the control board may be stuck and the board must be replaced.
• Check mixing valve: Ensure the thermostatic mixing valve is not stuck at a high setting; adjust or replace as needed.

Do not restore power until the cause of overheating is corrected.