Unit displays E04 and will not start; often associated with overheating or safety thermostat trip.

Safety: Isolate power and gas. Allow the appliance to cool completely before touching internal components.

• Check water flow through heater: Ensure inlet filters and strainers on the cold water connection are clean. Remove and rinse under running water.
• Descale water path: On older Chaffoteaux instantaneous water heaters, heavy limescale in the heat exchanger can cause local overheating. Isolate the heater, disconnect the water connections, and circulate a manufacturer-approved descaling solution through the heat exchanger using a pump. Rinse thoroughly afterward.
• Inspect overheat thermostat: Locate the overheat stat on the primary heat exchanger. If it has a manual reset button, press it once. Check wiring for signs of overheating or loose spade terminals.
• Check any internal circulation pump (if fitted): Some combi-type water heaters have a small pump. Listen for operation when there is demand. If the pump is seized or noisy, it may need replacement.
• Reset and test: After cleaning and resetting, restore power and gas, then test with a moderate hot water flow. If E04 recurs, the thermostat or heat exchanger may need replacement by a professional.