Any ideas on how to fix my Ferroli Optimax HE Plus 31 C combi boiler?Ferrol?

Ferroli Optimax HE Plus 31 C 3 years old from new. Have heating but no hot water, when I turn on hot tap the DHW mode operation symbol does not appear on the boilers LCD screen and the burner doesn't ignite. The Heat Exchanger to Flow Meter connection (39837170) was replaced as it had a tiny fracture and was spraying cold water everywhere as it is directly above the housing containing the cold water flow limiter, inlet filter and flow meter. Since that fault occurred no hot water, pcb board looks dry and in good condition, DHW temperature sensor is new. Live in a very chalky water area. Any ideas would be helpful.