The ECT sensor is in the cylinder head. It's for the computer.

If you do have a Fan Switch it will either be in the radiator or at the thermostat housing. 

But you may want to consider a faulty fan relay too.

Yours has 3 relays in the underhood fuse box to control the fan speeds and they're labeled something like Fan 1, Fan 2, and Fan 3.

In your case you know the Fan Relay 3 is working because the fans work in Low Speed.

That leaves Fan Relays 1 and 2.

If the Fan Relay 2 is working and the Fan 1 Relay isn't then, when the Fans were commanded into High Speed, one of the fans would run at High Speed and the other fan would stop working completely ((troubleshooting clue)).

If the Fan Relay 1 is working BUT the Fan Relay 2 isn't working the fans will stay in Low Speed even when commanded into High Speed.