Mod note: Moved to the Therm. forum.
FYI, the worst thing that makes most probes go bad is moisture. Check the location where the solid metal probe meets the cable. That's the location that if ANY MOISTURE gets into it, it works it way down to the end of the probe and then makes the insides go haywire.
When you can, put shrink tubing over that location (for good units) and then never immerse it in water to clean.
For bad units, we've talked about immersing the whole probe in hot oil. Hot oil forces the water out. The clean it up, and test. If it's good, then add shrink tubing.