Well, if the display is visibly damaged, as if the LCD has cracked and leaked its juice inside the glass, then it is done for.

Testing the probe is done with an ohmmeter. If the probe is NTC (negative temperature coefficient), which I *think* it is, then the resistance will go up as temperature goes down. If not, then the opposite is true. If you get no resistance (zero ohms) then the probe is shorted. If you get infinite ohms, then the probe is open (broken wire).

But it sounds like your display is trashed. Got a pic? If there's a big black blob in the display, then the LCD is cracked and the unit is history.

-tp