I think the one right on top isn't a sensor. It gets voltage to open the thermostat... its all computer controlled... if I find my old tstat I can tell you for sure. I should have it around somewhere
http://www.aveoforum.com/forum/f108/...29/index2.html

Looks like a semi common problem. The whole unit is only 130 though, its not super expensive like a BMW's electrically controlled tstat.