i am having problems with a sensor on a trane system. the initial call is that unit is freezing up. after several attempts to locate problem i realized the temp was reading 108f and it was only like 76f. i realize it works off resistance and was told to see if problem was shorted and put bulb sensor straight on board. the temp was reading accuratly. ran a new tstat wire to return and hooked up sensor. came back and temp was way off again. friend told me i might have used too large of a wire because it works off resistance. ran smaller wire no help now if i put bulb to board still reading is off. is board, bulb, or is something else bad. would appreciate some help thanks.