Outside temp sensor

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Hi, the outside temp reading is erratic and I cant find the cause. there are 2 sensors mounted in front of the radiator, one for the ATC and the other for the outside temp . Here are symptoms : when I first start the car, like in the morning, the reading is fine, it reads the actual outside temp, but after a couple of minutes driving, the temp starts dropping very rapidly into the negative zone and stays there...until the next day.... I checked the sensors and both measure the same resistance for a given temp.. I haven't noticed anything else wrong with the dash display. I know it's not absolutely necessary , but it's driving me nuts.. Any ideas? Thanks a lot