@jcj - The problems we're talking about are not the traditionally-accepted blockage of one or two sensors, but changes to the design post-certification that raise the risk of a triple failure.
You have two probes .. three probes .. certified or modified (and certified again) .. when they will find it in the right environment (air freezing or ice particles) the greater the chance that they will all be put out of function
So .. 2 probes .. double failure ... 3 probes .. triple failure ....
But if you have 3 probes and one is heated near is melting point .. you will have only a double failure
It's just to find how heat a probe near his melting point .. and able to continue to function normally ...