First I would try to clean it. Lay down in front of the car looking underneath the bumper. See that black canister shaped thing in the middle of the radiator? Try taking a clean towel and wipe the top of the sensor off. There is probably 9 years worth of crud on it. Get back in the car, start her up and drive around the block. If the temp reads ok then your done.
If not, its time to be a new sensor.
