Vaillant ecotec 937?

Firstly thanks to ltd for response reference no hot water. I have no hot water from my Vaillant ecotec 937 - it shows no Fault Code - the water comes through the hot taps "stone cold" but the radiators get hot as normal - the diverter valve was changed last week - aside from the sensors what other causes are there - I've been told a leaking / faulty diverter valve can have an effect on the main PCB is this true - Thanks