Ariston water heater shows E02 and shuts down; water temperature is unstable or cold.

Safety first: Isolate electrical power at the breaker. Confirm with a multimeter that no voltage is present.

• Locate the NTC sensor: On Ariston heaters, the NTC (temperature probe) is usually inserted into a dry well on the tank or attached to the outlet pipe, connected by a two-wire harness to the control board.
• Check connections: Remove the front cover and inspect the NTC plug at the control board. Reseat the connector and ensure there is no corrosion or moisture.
• Measure sensor resistance: Disconnect the NTC and measure resistance across its pins. At room temperature (~25°C), many Ariston NTCs read around 10 kΩ (check service manual for exact spec). If it reads near 0 Ω (short) or OL (open), the sensor is defective.
• Inspect cable: Follow the harness from the sensor to the board. Look for cuts, pinched sections, or rodent damage. Repair or replace damaged wiring.
• Replace NTC sensor: If faulty, remove the sensor from its well or clamp, install a new Ariston-compatible NTC probe, ensuring good thermal contact and correct routing of the cable.

Restore power and clear the error by cycling power. If E02 returns with a known-good sensor, the control board's sensor input circuit may be defective and require professional replacement.