You could do what I did and do a Google search. You say a Ferroli boiler but didn't bother to tell us the model, so this is a bit of a guess.
Here's what I found inside 60 seconds:
http://www.howtomendit.com/answers.php?id=307176
https://uk.answers.yahoo.com/question/index?qid=20071123120523AAYievC
http://www.plumbheat.info/repairs-common-faults-info-for-ferroli-boilers/
http://www.mrboilerrepairs.co.uk/fault-codes-Ferroli-DOMIcondends-HE26C
XXX, January 2016