Let the car warm up to operating temp and open the 7mm bleeder on the t-stat assuming you have a 3800 engine. If you have a 3.1/3.4 there are two bleeds one by the t-stat neck and the other is by the water pump they are both 7mm. More than likely your sensor has crap in it, you can clean it out or just unplug it it is on the top right of the rad.