Find a competent engineer who is able diagnose the fault to be the pressure sensor and explain why. Other wise it will be the build up of sludge and debris from the radiators in boiler that is affecting the sensor - best of luck
Dosta
September 2008
From Vaillant manual.
F.75 No pressure rise was detected on turning on the pump Water pressure sensor or/and pump defective
Too little water in appliance; check adjustable by-pass;
May be a stuck or faulty pump also check pressure guage is at least 1 bar
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