Digital Tesy water heater does not heat; display shows E01 shortly after power‑up.

Safety first: Switch OFF the circuit breaker and verify no voltage at the heater terminals with a multimeter before opening any covers. Water inside the tank may be hot—allow cooling time.

Steps to check and fix:

• Access the electronics: Remove the front plastic cover or lower service panel of the Tesy heater (usually held by screws). Locate the main control board and the NTC sensor cable leading into the tank.
• Inspect the NTC wiring: Look for cut, pinched, or burned wires between the NTC sensor and the control board. Reseat the connector on the board and at the sensor (if accessible).
• Measure the NTC sensor: Disconnect the NTC from the board. Using a multimeter on resistance (kΩ range), measure across the sensor leads. A typical Tesy NTC will read between 5–20 kΩ at room temperature. If it reads OL (open) or 0 Ω (short), the sensor is defective.
• Check the connector and board pins: Clean any corrosion with contact cleaner. Ensure the connector locks firmly into the board.
• Replace the NTC sensor if faulty: Order a compatible Tesy NTC temperature sensor for your exact model. Remove the old sensor from its well or pocket in the tank, install the new one with the supplied gasket or thermal paste if specified, and reconnect the wiring.
• Reassemble and test: Refit the cover, restore power, and reset the heater. Confirm that E01 no longer appears and that heating starts normally.

When to call a technician: If the NTC tests good but E01 persists, the control board’s sensor input may be damaged and the electronic control module will likely need replacement.