Solenoid valve is jamming open or has debris trapped under the valve seating. It is usually a waste of time trying to stop them sticking so unless you can see anything that is obviously jamming the valve seat open, it is best to fit a replacement unit
Turn water off at the isolator. Isolate the power. Remove top of machine to access the solenoid valve.
Remove the power leads and remove valve body.
Then Google your machine model to find spare part outlets. Try www.espares.co.uk as a start
Cledlynwader, October 2008