F75 fault with Vaillant when pressure is okay

[diynoob123]

Can someone help me understand what could be causing this fault?
It only seems to happen when the pressure in the boiler drops below around 1.5 - 1.7 bars otherwise it works fine.
The boiler is a Valiant Eco fit pure 835 and the building is a 3 storey house with 10 radiators.

Thanks

 

[ShaunCorbs]

The pressure switch has a min of 1.5 bar so you need to be above this

 

[Last Plumber]

You need a gas safe engineer to it.
This is a really easy fix and shouldn't cost a fortune.