i have a valiant eco plus and keep getting a f.75 fault ?

i have a valiant eco plus combi boiler and keep getting a F.75 fault which i have looked up i keep filling up and make sure that the presure on the digital readout is at about 1.5 bar but when heating system comes on when you look at the manometer it s going into the red but if the pressure is at around 1.3 bar it starts to go in to the fault code