You do not say the make or model of your boiler - Ferroli use this error code - a previous answer on here from Taffyplumb in April 2011 said this...
This occurs when the heating is running and you turn on the hot water for a considerable tim..ie bath or shower... it is just an overdemand on the system...it doesn't really mean anything!!
i.e Boiler temporarily unable to heat water due to over demand - a feature of Combi boilers I believe...
Good Luck..., February 2017
