sensor is where anhydrous7 described. You can easily remove that little grille and pull the sensor out and unplug it. Check it's resistance with a multimeter at different temperatures. There are specs for these temps in the manual, I forget what they were....maybe near 2000 ohms? Anyway, I did this once on my 300SD and the values matched pretty close. At least try and see if it responds by leaving it outside to get cold, and then the resistance should change when you breathe on it.