how can i solve hotwater problem on vailliant torbomaxvuw?

my vailliant turbomax will not give hot water when switch to hot water and the hot water light wil not come on if i put the centrel heating on the water will come and the hot water light stays of..i have change 3 way valve and micro switch also acid fluse the small heater exchanger but still no luck ..i have also check all fuse,and bleed system .3 times..any one can help..