Ideal logic heat 18 not working

[Flaxxx]

I'm having issues with my ideal logic heat 18, it was showing fd error code so I replaced the flow sensor also thoroughly flushed ot the heat engine and pipe work, it will run fine on service mode but will not fire up unless the heat exchange is tapped lightly then will shut down when tapping stops, any help gratefully recieved

 

[SimonG]

Get yourself a good gas safe engineer to have a gander.