Hello. the first thing you need to determine is if the solenoid is getting 24 volts when the furnace turns on, you should be getting 24 volts at the solenoid valve. If not you need to start at the transformer and see if you are getting 24 volts there, if not then most likely the transformer is bad and must be replaced, if you are getting 24 volts at the transformer then check the wiring and make sure it;s tight. also check the humidistat and sail switch/ current sensing relay to make sure they are working properly. If you are getting 24 volts at the solenoid valve and is not allowing water thru than it must be replaced.