FERROLI OPTIMAX 25C OVERHEATING?

When I turn on the hot tap the boiler fires up then a few seconds later it overheats and I get F3 fault code, the pump is running and it has a new flow sensor as this is what the fault finding guide stipulates, I have noticed that the diverter valve was switching to heating when hot water is demanded and hot water when heating is demanded, the diverter valve has also been swapped to see if it was that but still the same fault, the pcb has also been test swapped and so has the hot water flow meter still to no avail! any advise would be greatly apreciated :)