I think by now we all know that the tempe wireless temperature sensor is defective. Garmin is working on producing corrected units and, if you reported yours to Product Support, you should get a new one when they are available. In the mean time, here is a description of how the tempe works - in case you haven't figured it out by now.
From Montana Dev Team:
I’ll give you a quick and basic example of how the feature works. I put the tempe under a light bulb and it reads 100°F at 10:00:00 AM on Thursday. I move it away from the light bulb and it somehow magically cools to 75°F at 10:00:01 AM on Thursday. It remains at 75°F until I heat it up to 85°F at 11:00:00 AM on Thursday. It then cools down back to 75°F and remains at that temperature. At 9:59:59 AM on Friday, the 24hr max will read 100°F. At 10:00:01 AM on Friday, the 24hr max will read 85°F. Because the 100°F temperature was greater than 24 hours in the past, it is no longer the 24hr max.
The 24 hour min/max temperatures are saved on the tempe. That allows the 24hr min/max to be preserved even if you turn off your handheld or walk out of range to the tempe, and it gives you more flexibility when using the tempe. As a basic example, you could leave it outside far away from your handheld device in the evening and then return in the morning with your handheld to figure out how cold it got the previous night. The only way to “reset it” and clear the 24hr min/max is to remove the battery from the tempe and wait several seconds. The tempe will then forget everything and start from scratch.
I'm afraid I'll be somewhere between home and Tierra del Fuego when the new tempe ships - being optimistic.