Same Issue
I am having the exact same issue on my G2. I have it in a repair shop currently and I will let you know what they find out. Have you dropped or damaged your phone in any way? Mine only started doing this after I dropped it and cracked the screen. I have the screen repaired and then that error message started showing up. I figured I would reset the phone and see if it made the error message go away, well when I turned the phone off...it wouldnt turn back on. so i took it to the repair shop and they fixed it!
It turns on no problem and the new screen is perfect. well, that was for about 2 days, then the error messaged popped up again, I reset the phone, it powered right back on and the message was gone. the next day, same Low Temp message AGAIN! I reset the phone and it would not power back on. Its at the repair shop again, 3rd time in a week, to try and figure it out. I'll keep ya posted and if you fix yours let me know!
-Adam