Display shows E03 and the water heater shuts down; fan may not start or stops quickly.

Safety: Turn off power and gas. Do not operate the heater with a suspected flue blockage; risk of carbon monoxide.

• Inspect flue terminal: From outside, check the concentric flue terminal for obstructions such as leaves, bird nests, or ice. Clear any blockage carefully.
• Check air intake path: Ensure the air intake (on room-sealed models, the outer flue ring) is not blocked. For open-flue models, ensure the room has adequate ventilation and that grilles are not covered.
• Check system water flow: Low flow can cause overheating. Confirm that inlet filters on the cold water connection are clean and that hot water taps are fully open. Descale the plate heat exchanger if flow is restricted.
• Inspect flue gas thermostat / overheat thermostat: Locate the safety thermostat clipped to the heat exchanger or flue hood. Check for signs of damage or loose wiring. Many are manual reset: press the reset button if present. If it trips repeatedly, do not bypass it; find and correct the root cause.
• Check fan operation: With the cover removed and power restored, start a demand and observe if the fan runs smoothly. If the fan does not start or is noisy, it may be failing and should be replaced.

If E03 persists after clearing the flue and checking flow, contact a qualified technician to test the flue gas thermostat and fan.