Greenslave ii

[Rubix]

Good Evening All

I have a Greenslave ii boiler, the pressure gauge is showing nothing so I filled the system and the prv started passing. The gauge does not move from zero.
The central heating seems to be working fine, I had the boiler serviced in August, there is a slight hissing sound when boiler is running not sure if this is the fan.

 

[Rubix]

Any advice would be greatly appreciated before I call the engineer back.