Water heater does not heat at all; no error code (mechanical thermostat model, indicator light off).

Safety first: Turn off the main breaker before opening the heater’s electrical cover. Verify absence of voltage with a multimeter.

Steps:

• Check supply: Confirm the dedicated breaker is ON and not tripped. Reset if necessary and see if it trips again.
• Inspect terminal block: Remove the cover and check the mains terminal block for loose screws or burned insulation. Tighten and replace damaged parts.
• Test thermal cut-out: Locate the manual reset thermal cut-out on the thermostat or element flange. Press the reset button. If it clicks and power returns, monitor for recurrence.
• Test thermostat: Disconnect power, remove wires from the thermostat, and check continuity when the knob is set to maximum. Replace the Tesy mechanical thermostat if open.
• Restore and test: Reassemble, restore power, and verify that the indicator light comes on and water begins to heat.