We don't have a second AC, but Larry's right about what the sensor looks like. Ours was wired for two and has the sensor screwed to the ceiling just as he said. It's pretty easy to find, but if you can't, I wonder if they left it in the ceiling plenum next to an AC duct from the rear one, and that's why it's reading so cold. If it were mounted on the ceiling, there's no way it could be getting those temps.
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