If the unit is OK then it can only be either the float switch or the solenoid valve.
Put your hand inside tank,and see if the float arm is stuck in the full position,that is probably what's happened and it's telling the controller the tank is full
Good first starting place Colin
How do you know the float switch is ok? How have you tested it? Is there power from your controller to the valve. The best way to check this is with a multimeter or test lamp.
The first thing I would test is the float switch after you have tried Colins suggestion. A multimeter will soon tell you if you have a broken circuit or not.
You may try to make a jumper cable from a small piece of electrical cable and bridge the two wires at the float switch. If the float valve is faulty then the solenoid valve will open.
If nothing happens then you need to check if the solenoid valve works. You will have to use 2 lengths of wire, 1 positive and the other negative. You can use any 12v battery provided the solenoid valve is 12v. It will have its voltage on the side.
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