Ferolli optimax boiler keeps saying F 37?

Ive had numerous engineers now to look at my boiler, the cost is escalating to the point where it would have been cheaper to buy a new boiler. So far ive had the compression fan changed, new actuator switch, new PCB and now most recently a new pressure switch, yet still F 37 crops up. The pressure has been checked, is at 1.5 bar, does any1 else have any ideas??? much appreciated