how can i fix my ferroli cobi boiler?

are you getting water leaking form the overflow pipe outside?
If so it may need cleaning or replacing, bet money your serviceman used the 3bar safety valve to drain the boiler, then some crap in the water has prevented the valve from seating properly again allow a dribble of water to leave your system and resulting pressure drop.

When will people learn, the safety vavle is not a boiler drain point!

T
May 2008