Vailliant turbo max plus 824e?

vailliant turbomax plus 824e combi boiler, intermittent fault on domestic hot water supply, fails to come on when called for sometimes, have changed all three NTC,s flow, return and warm start it was all good for few weeks but now started doing it again, when it fails the display show f24. Also re pressurised the
xpansion,as it was less than it is supposed to be.

Any suggestions please