I know I'm dredging up an ancient post but just in case anyone else uses one of these...
The probes do fail.
They are however, fixable.
The trouble with them is that they aren't particularly waterproof and don't perform well in an ultra-high humidity environment like your mashtun. The water gets into the probe and causes some very freaky effects with it's electronics. Normally will result in your thermometer reading really high.
The fix is simple but temporary.
Take a pair of pliers and grip the exposed part of the black seal where the wire goes into the probe. Pull. REALLY Pull. The business end of the probe will come out. Dry it off and leave it somewhere dry and hot to drive off any residual moisure.
Put the metal casing of the probe in the oven next time you have a roast on. That'll dry it out nicely.
Reassemble and test.
Get frustrated next time you kill it and vow to buy a more suitable one...