I have the same experience with the sensor as other posters, i.e. never seen a failure!
I flew the 747 Classic for 23 years on worldwide, including arctic, routes. I cannot recall (but, due to great age, my memory may be faulty!) ever encountering a U/S fuel temperature gauge.
However, naive question, how can you be sure that a single temperature gauge is not over-reading?
I am certainly not pushing any theory that the single fuel temperature gauge caused the accident. But it is logical, in my opinion, to question what common components there were that caused independent ETOPS engines to fail to provide the required thrust at virtually the same time, thus causing a catastrophic accident.
What other common components were there?
Regards
Stoic