Display shows E03 and the Ariston water heater does not start heating; sometimes trips safety thermostat.

Safety first: Turn off power at the breaker and allow the heater to cool completely before working on it.

• Reset safety thermostat: Remove the front cover and locate the manual reset safety thermostat (often with a red button). Press to reset. If it immediately trips again, do not keep resetting—find the cause.
• Check water flow (instant heaters): Ensure inlet valve is fully open and filters/aerators are not clogged. Low flow can cause overheating and E03.
• Inspect for limescale: In storage heaters, heavy scale on the element and tank walls can cause hot spots. Drain the tank and inspect the element. Descale or replace the element if heavily coated.
• Verify thermostat settings: Ensure the set temperature is not excessively high. Reduce to 50–60°C to reduce stress on components.
• Check control board relay: If the element continues heating even when the thermostat should be off (overheating), the power relay on the control board may be welded closed and requires board replacement by a technician.

If E03 recurs frequently, schedule a professional inspection to check for internal scaling and verify all safety devices.