You are correct, there is one "sender" and one "sensor". The "sender" is what makes the needle on the gage move. I believe that is located on the block some where. The "sensor" is located in the thermostat housing. It's the one that talks to the ECM. Make sure the plug is clipped on to the sensor.(that's an easy first check) Sometimes when they change the thermostat, they forget to plug the sensor back in, you have to unplug it to change the thermostat. If that doesn't work, you may want to try replacing the sensor.

BUT!!!

Have you tried checking just the fan? All you have to do is disconnect the fan and hook up a 12v source to it(ie battery in the car) If the fan spins when you hook up power, the fan is good, if not, the motor maybe burned out. That's an easy and free check as well.