Are you sure that the controller is actually keeping the solenoid open when it drops to 18? There is often an overshoot (or "undershoot") of your setpoint due to thermal lag in the system. For example, when the probe detects 19, the controller should shut off. But the beer in the tank is already on the way to 18 and the convection currents, conduction to the probe, and cold glycol caught in the jackets very often (if not always) contribute to an undershoot. I've seen most small breweries experience this problem with any type of controller/probe. There should be a green light on the controller that shows the status of the solenoid. Lit when energized. Otherwise, programming this controller is quite easy and I doubt you've screwed up anything. Sometimes you just have to live within the limitations of your equipment. I do with my A419. Fortunately, I have them on my walk-in and my glycol chiller--not my fermenters. Good luck!