Main 30he flashes low pressure or pump

[Pjar]

Hi
Please help I have a problem with the boiler main 30 he flashes low pressure or pump usually if it is switched to central heating I note that sometimes central heating and hot tap water go back to normal by themselves or after turning the prv.
I checked or fixed:
reconditioned diverter valve not leaking.
water exchange plate looks good but I will clean the chemistry.
expanding vassel air pressure is 0.8bar with water 1.1bar no leaking from valve.
central heating pressure is over 1.5 bar no more than 2.0.
does anyone know where is the problem with this boiler
many thanks for help

 

[Stigster]

Was the expansion vessel air side checked or refilled with zero pressure in the system? I have found that some plumbers have been charging or checking expansion vessel pressure without realising they need to drop the system pressure to zero before doing so otherwise you get a false reading.