I'm slightly amused that I am the only person who responded to my question, but oh well. If no one knows the answer, that's just how it is. Anyways, I thought I'd post the solution so if someone else has this problem in the future they can have some help in the archives.

There are three possible sensors that cause the P0112 code on an S90:

1) the temperature sensor in the cabin climate control, located just behind the passenger kick panel. I replaced it, and the code did not go away, though the A/C seems to be much more consistent.

2) There are two air temperature sensors just below the radiator support that are often falling out of their housings. My dad taped them up years ago, but one of the connections had come loose. Fixing the connection cleared the code.

I'm now running without a check engine light!
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1997 850, 1979 242GT, 1990 740GL