I have a 1999 3500 van. Recently the temperature display on the rear view mirror has been acting strange. Sometimes when I start it will display the correct temperature, then suddenly go to -40 degrees. It will then slowly work its way back up until it displays the correct temperature again. This can take 15-20 minutes. Other times when I start up it will immediately display
-40 degrees, then work its way back up to the correct temperature. On long drives this behavior goes on back and forth. Any suggestions on how to correct this irritating problem?

 

Did you try replacing the outside temperature sensor?