In Terminator 2: Judgement Day, the T-800 tells Sarah that he "cannot self terminate". I had always assumed that this applied to all Terminators.
But as Jason Baker points out in his answer to this question, it actually appears that the T-850 in Terminator 3: Rise of the Machines does something that could be considered self termination, albeit in an attempt (ultimately successful) to kill the T-X.
But one might argue that the T-850's "death" was less a matter of self termination, and more of a sign that Terminators are perfectly willing to allow themselves to be destroyed if it helps them complete their mission. For instance, the T-800 is actually the one who suggests that he should be melted down, because his mission was apparently twofold: 1. Protect John Connor (and obey his orders). 2. Prevent Skynet from coming online. The latter objective is the one that compelled him to let Sarah destroy him.
And in the first movie, the T-800 is so obsessed with killing Sarah that it ignores the hydraulic press it is crawling through to reach her. Again, this suggests that Terminators will disregard their own safety in order to complete their missions.
But the T-850's "death" is still slightly different from either of these other examples. The first T-800 didn't actively kill itself, it merely put itself in a situation in which it could be killed, because it was trying to complete its mission. And the second T-800 had already completed its mission, but was unable to self terminate, and needed Sarah to do the dirty work, so to speak. In neither of these cases did the Terminators kill themselves; rather, they both allowed themselves to be killed (in the first case, willingly, and in the second case, unwillingly).
The T-850, on the other hand, did actively kill itself. Granted, it killed itself in order to kill the T-X, thereby completing its mission, but it is hard to deny that its actions amounted to self termination.
So, as I have already said, we might assume that Terminators are capable of self termination if it allows them to complete their missions, and this might be all the explanation necessary.
However, there is another possibility: shortly before the T-850 self terminates, it is corrupted by the T-X and reprogrammed to kill John. It is seconds away from killing him, when John reminds him of his mission: to protect John Connor. This causes a sort of paradox, and the T-850 manages to shut itself down, thereby protecting John, and later overrides the new programming and reboots itself. This suggests that this Terminator has some degree of willpower, despite his statement to the contrary:
Desire is irrelevant. I am a machine.
We know that the T-850 is an upgraded version of the T-800, although the exact nature of the upgrade is not clear. It seems at least possible that the upgrades allowed this model of Terminator to self terminate.
Are either of these explanations correct, or is there some other option that I am missing? Did the T-850 self terminate, and if so, how was it able to do so?