I have an underground sprinkler system. A few days ago a valve got stuck "on" with a loudly buzzing solenoid. I replaced the entire valve with a new one and wired it in, tested, and all was well:
Today the zone whose valve I replaced randomly turned on even though the program is not set for this time. Turning the system from AUTO (schedule) to OFF successfully closed the valve (and stopped the sprinklers), then setting it back to AUTO kept the valve off... for now.
Typically when a valve is stuck on there's either debris in the line or the solenoid went bad. I ensured both are solved with my replacement valve. So, given that the new valve is still exhibiting quirky behaviour, what could possibly be going on? It must be receiving power to be on at this point so could the controller be going bad? I've had the controller since we moved in 3+ years ago and it has worked perfectly.