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..

jj
October 2008