Testing AZ temp guage in pan of water...help

try to test my "autozone" temp guage out of the car to see if it's accurate in a pan of boiling water w/ thermometer in there. I pulled the guage and the wiring and sensor and hooked it all up. I have the (+) from the guage going to the (+) side of battery; the (-) side of guage goes to (-) side of battery and the "source" at the guage goes to the temp sensor, which is suspended in a pan of water. Seems cool right? The guage won't move at all. Is there something I don't know? I've checked voltages and I have 12.7v at the (+) side of the guage using the (-) side of the guage as the ground, so I know the connections SHOULD be good. What gives?

right on the money, jrichker...thanks The guage was about 10* too high up until about 210*, then it was pretty much accurate. The water only went to 220* before I shut it down due to time restraints. I kinda wish it was wrong lol. 220* should be sufficient since the car SHOULDN'T get that hot anyway...unfortunately it does. I'm gonna try it out one more time and post questions after that.