Every so often a zone will stay on long past its time to have shut down. Many times this is due to the solenoid valve or zone valve being stuck in the "open" position. You try to stop the zone from being "on" by turning the controller to "off" and the zone stays "on". Next, you remove the plug from the controller thinking that it can't get any power and yet the zone stays "on". The problem is hydraulic not electrical. Typically it is because there is dirt and debris in the zone valve preventing it from closing even though the controller has ceased sending a signal for the valve to be "open" or "on" the valve can't physically close. The only way to close down the zone is to shut off the sprinkler main valve.
Once the water is off to the sprinkler system then the valve can be taken apart and cleaned. Over the years, I have found nuts, bolts, rocks, glass, and unidentifiable objects in all kinds of solenoid valves. These objects work their way into your plumbing system and then into your sprinkler system from the water main out in the street. It happens every spring when people start using more water and on really hot days when everyone is watering their lawn. The more water the public uses simultaneously the more turbulence there is in the water mains. Therefore, the more debris is stirred up and distributed to everyone.
The "solenoid valves" have a rubber diaphragm and small ports inside that create a pressure differential making the valves open and close using low voltage electricity and controllers. Once those ports get clogged or dirt settles on the diaphragm then the valve cannot close down. Sometimes if you turn the water on and off to the system that will dislodge the debris and it will move downstream to the sprinkler heads. Most of the time I will need to come by and take the valve apart and remove the debris. Sometimes the debris ruins the internal components and they will require replacing. On older systems, the solenoid valves may not be able to be serviced and then the entire valve will need to be replaced. If you live in an area that has an older water main infrastructure then I recommend installing a filter before the manifold and zone valves to catch all the debris. There are several very good filters on the market that would do a great job preventing problems in your sprinkler system. For more information or questions about this problem please call 631.423.2211 or email me.