One clarification, Low. You said
"One easy way to rule out the UI is to disconnect it from the main control with the power disconnected. Then plug it back into the wall. If the control cycles the water valves, it's good. If it still does nothing, replace the main control."
So, If control cycles water valves, is UI bad? You said it's good. If not, replace main control. How do you know if UI is bad, vavles cycle or not?