Display shows E05 and heating stops; unit may start and stop repeatedly.

Safety first: Isolate power before accessing the control compartment or sensors.

• Locate the inlet sensor: On Hewalex pool heaters, the water inlet NTC is typically mounted in a sensor well or directly in the inlet pipe. Refer to the unit’s wiring diagram for exact location.
• Inspect wiring: Check the sensor cable for cuts, abrasions, or rodent damage. Ensure the plug is firmly seated on the control board.
• Check resistance (if equipped and qualified): Disconnect the sensor and measure its resistance with a multimeter. Compare the reading to the Hewalex temperature–resistance table at the current water temperature. An open circuit or near-zero resistance indicates a faulty sensor.
• Dry and reseat: If there is moisture around the sensor, dry the area and reseat the probe and connector.
• Replace if needed: If readings are out of range, replace the inlet NTC sensor with a Hewalex-compatible part.