My boiler would show F75 fault code about once a month after it was 2 years old & out of warranty. I was told to replace the CH pressure sensor as they have a tendency to suffer from ionization (I think I've spelt it right). Vaillant part No 253595. I was also told the new valve is a mod (old valve brown/orange. New valve black and a bit bigger.
I have fitted new valve, flushed & refilled system, no problems since - 13 months ago.
Just recently I have fitted a magnetic filter in the system to be on the safe side.
havago
August 2010