I have no real idea, especially because the outside unit is about 10m away and shows ~700ppm. I will try swapping them to see whether it's a calibration issue,

I have no real idea, especially because the outside unit is about 10m away and shows ~700ppm. I will try swapping them to see whether it's a calibration issue, but right now the outside unit is at work with me so I can open it up and take photos.,