How can i fix my ariston micro genus 23?

Hi,My pressure reading when off is almost zero(first line in red)boiler will turn on and run fine but then the needle will show 2 to 2 and a half bar if i turn up the heating the needle rises to 3 to 3 and a half bar but the pressure relief valve does not open,if the heating is off and the hot water is turned on the radiators start to get hot the boiler does not make any funny noises or fail to function i put in fernox cleaner and inhibitor about two years ago the boiler is about 6 years old and never had any problems,is this something i can fix myself?appreciate any advice given,thankyou.