Alpha [E03] fault codewhat it means and how to fix it

A condensing boiler is supposed to send remarkably cool gases up the flue — that is the whole point of the design, because heat that goes up the flue is heat you paid for and did not use. E03 means the flue gas sensor has recorded a temperature well above what the appliance should ever produce. The reason is nearly always that heat is not getting into the water where it belongs, so it leaves through the flue instead. Scale or sludge coating the inside of the heat exchanger is the classic cause, and it is progressive: efficiency degrades for months before the sensor finally trips. The secondary concern is the flue itself. Plastic flue systems are rated for the temperatures a condensing appliance normally produces, and sustained overtemperature is exactly the condition they are not designed for. That is why this code locks the appliance out rather than simply logging a warning.