its either the diverter valve is faulty and may need replacing as when u open the tap it pushes forward a pin that in turn presses a micro switch to activate the boiler for hot water. if the valve is faulty the pin does not retract, or if the micro switch is faulty it still stays pressed on. if those 2 fail then it would be your pcb is faulty they can be checked easily
kash, December 2012
