I have the same problem on an Ariston MicroGenius II 28 MFFI. My boiler has to be reset several times per week, sometimes several times per day, then might work for a fortnight. Ariston approved engineers have replaced the circuit board twice, some sensor or something and it's still happening. Thank goodness I took out the extended warranty!
Duncan
March 2009
I had this problem - my boiler was showing the A03 error and stopped working altogether.
It means that the input temperature was exceeding 103 degrees because the pump had broken. An engineer came round today and replaced the pump, lowered the water pressure and that did the trick.
So, the problem is most likely a faulty pump.
Phil Brown
February 2009
I have this same problem, did u find the answer please?
roxky
January 2009
I get the same message, press the reset button, and 10 mins later the same message appears! I contacted Ariston earlier in the week and they promised someone would call me within 24 hours but no-one has. I have no heating or hot water!