[rondeo]

My expertise re the dct450 is limited to reading the manual by ATSG and Ford.

I suppose the GISS sensor is a hall effect device. In other word an electromagnetic sensor. From a purely speculative point of view it seems plausible that metallic contamination of the transmission fluid might interfere with the working of the sensor.

In any case there's one sure way to see if there's any fluid borne deposit of metallic particles: take a look, unfortunately.

Another permutation is that the hall effect sensor is dead, that would be a permanent malfunction.

An intermittent contact or bad connection may also be suspected.

Surely your local Ford dealer can tell you what the GISS sensor part number is?

Looks like you have a good handle on the issue anyway!