On later models, with the fuser out of the machine, the thermistor is readily accessible; no disassembly of the fuser is required. The Optra S and some early 'T's needed the fuser's top cover to be removed in order to replace the thermistor.
The thermistor is at the upper front of the fuser, near the left side end. On later models it's held in place with a single M3x10mm pan head screw w/captive washers. It has a two-conductor cable that runs back to the fuser's PCA. Early machines had pushnuts for cable restraints -- the pushnuts can be coaxed off by using a scriber point as a fine prybar. Later machines have open hook-restraints.
A good thermistor exhibits a room temperature resistance reading of approximately 400 kohms. The resistance falls with rising temperature (negative temperature coefficient). You can see the effect on an ohmmeter just from the warmth of your hand. The thermistor's resistance provides the engine controller's logic with an analogue of fuser temperature. An open (infinite resistance) thermistor renders the fuser inoperative; hence the fatal 924 error very early on in the printer's POST.
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