if its just dripping then the diaphragm has failed. need replacing new valve.
if its running at full power the solenoid has failed and needs a new solenoid.
if you get one you have to buy the complete valve/solenoid as one unit.
but i can get just the solenoid on its own half the price and saves less time if its only the solenoid failed.
hi-spec plumbing & heating (essex), July 2009