low coolant light Buick century 2002

The lights stays "on" after start up, sometimes for a few minutes, sometimes for over an hours. Coolant level ok in overflow tank,

I am assuming the sensor is b/o, but where it is located ? in the tank, in the rad ??

Any other thing to check ? What is the value of a good sensor ( Ohms) Or how can I make sure that its the sensor and not something else, eg wiring ?

Thanks