Display shows E04 and heater does not produce hot water or shuts down quickly.

Safety: Turn off power at the breaker and close water supply. Allow the unit to cool completely before working on it.

• Check for flow restrictions: Clean inlet filter screens and any external strainers. Ensure all isolation valves are fully open and that aerators on hot water taps are not clogged.
• Inspect safety limiter: Remove the cover and locate the safety temperature limiter (often a small round or rectangular device mounted on the heat exchanger with two wires). Some models have a manual reset button on the limiter—press it firmly if present.
• Look for scale and overheating signs: Heavy limescale on the copper heat exchanger or discoloration may indicate chronic overheating. Consider descaling the unit using a pump and descaling solution according to the service manual.
• Check wiring: Ensure the limiter’s wires are secure and not burnt. Reseat connectors on both the limiter and control board.
• Test operation: Restore water and power, then run hot water. If E04 returns immediately or the limiter will not reset, replace the safety temperature limiter with the correct OEM part. If overheating persists, have a technician inspect for internal blockages or control issues.