have same boiler same problem. Just been serviced and engineer did not have an answer, checked pressure in expansion tank and fine and no water leaking in from filling pipe.
In the manual it says F75 no pressure rise was detected on turning on the pump - water pressure sensor and or pump defective, air in heating system, quick bleeder defective, too little water in appliance, check adjustable by-pass, connect external expension vessel to return.
No solution as yet. I resst boiler and it works for a week or so before the code comes back.
helpful
January 2010
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