The first time this happened was after it had rained. I was driving 65-70 mph and the temperature gauge in the dash drops down to cold. I checked the coolant temperature in the message center and there were three dashes where there should be numbers. The air conditioner compressor also will not engage when this happens and will not make the air conditioner work. The problem lasted two or three days and then the temperature gauge magically started working again. It's not too much of an issue when it doesn't work, but it may be linked to the engine protection system in case the engine overheats, and if the car doesn't know what temperature the coolant is, it'll more than likely run the engine hot and do some major damage.