sounds like you're testing the Fan switch, are you sure its the coolant temp sensor?
The dash gets its info from the Engine management, if there's no fault (dash lights), then sounds like sensor.
If the wiring was duff, short/open circuit, then the engine managment would light up.
Or faulty dash.
I was under the impression that the coolant sensor also controls the fan. The sensor I was testing is located underneath the vacuum servo unit.