[kylerush]

I live an hour drive north of New York City in a small, rural town of about 2,000 people. Large lot sizes, not much going on pollution wise. The lowest my CO2 has been so far is 659. This is when I open the windows for 30 minutes. I haven’t tried anything more aggressive than that. Either our rural, small-town air is worse than NYC for some reason, or the NYC air isn’t that bad? Very interesting.

[badpun]

Try opening your window for an entire day (e.g. when you're away) and see the result. If it's still above 600, my guess is that the sensor is crappy. Also, apparently some sensors recalibrate themselves against the lowest level they've seen in the past X days - so ventilating heavily may recalibrate it.