1997 Ford F150 temp gauge for engine reads cold

i bought this trucka coupe weeks ago. i was told by the dealership that the temp red hot until they put a thermostat in it and now it read ice cold all the time. they told me to look into some kind of sending unit for the heat. i had a mechanic check it out and he repeated the idea. i just don't know what it is or if i can fix it myself. any help?

thanks,

jon