You've not told us the exact model number of your boiler, but an educated guess and a quick search found this information on an A03 fault code:
"High limit protection – flow temperature sensor not active, incorrectly located or no system circulation. Check the position of the flow sensor or check pump and radiator valves"
With that advice from the manufacturer, I'd first look at the circulation pump if you have bled the radiators.
As I say, this information may not be relevant to your specific model, but in general, a Ferroli A03 code on most of their boilers points to a fault along the lines as that quoted above.
Rupert, February 2020
