I can't tell you the location of the sensor, but about 2 years ago, after an oil change at the dealer on my 2006 G6, when I drove home the temp display was stuck on a temperature reading that was abnormal for that time of year. It seemed to clear up on its own after a day and did not happen again until today.
I just drove out of the same dealer after an oil change, and the display is stuck at 16 F. It is 32 F outside. I called the dealer and as usual, they know nothing about this problem.
I will watch it for a day or 2 and if it doesn't reset, I will take it back. It is hard to tell if it is actually the sensor or perhaps the program that samples, reads and then displays the sensor measurement.