Impala temperature gauge problem

Yesterday driving on the freeway, the temp gauge suddenly dropped to the cold position. I pulled over and noticed the coolings fans were on high. Outside temp was just above freezing,and plenty of heat coming
from the heater, so I figure the sensor must be bad. At home I pulled the plug of a sensor located near the thermostat and measured 450 ohms. A few hours later it was 1200 ohms. There was 5V at the plug. I think the sensor is good, and the check engine light not on. Is there another sensor that feeds the gauge? I've looked around and nothing obvious. Any help would be appreciated.
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What year car and engine!!

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