Product review (4 stars): it seems it is reliable; when I came back ...
it seems it is reliable; when I came back from vacation with the family, the CO2 reading was low ( in 500 range), which is expected; the house was vacant.  then next day it jumbed to 700 range, which is also expected as my house is tightly closed most of the time.  However, surprisingly after a month it dropped to 400 ranges with nothing changed except the weather.  So here, I thought there is something wrong. But, when some visitors stayed over night, it jumbed to 900 which made me believe  it is working correctly.