Display shows E03 and heater does not start heating; sometimes occurs after overheating or dry firing.

Safety: Turn off power and allow the heater to cool completely. Hot water and steam can cause burns.

Steps to reset and prevent E03:

• Check water supply: Ensure the cold water inlet valve to the Eldom heater is fully open and that there is water pressure. Open a hot tap to confirm water flow.
• Reset the safety thermostat: Remove the lower cover. Locate the manual reset thermal cut-out (often a small device with a red button on the element flange or near it). Press the button until it clicks.
• Inspect for limescale: If the heater is older or in a hard-water area, remove the element flange and inspect for heavy scale on the heating element and magnesium anode. Thick scale can cause overheating.
• Descale if needed: Carefully chip away scale or soak the element in descaling solution if it is still serviceable. Replace heavily scaled elements and anodes.
• Refill and bleed: After any work, close the drain, open cold inlet, and open a hot tap until water flows steadily with no air before restoring power.

Call a technician if the thermostat trips repeatedly despite proper water level and descaling; the control board or thermostat may be defective.