Up to that point, I also assumed the meter was broken, but if it still shows reasonable values outside, I'm afraid it's the house that's broken.
Check the ventilation system to see if it's working properly. It may need to be cleaned or replaced. Maybe have some plants in your home. Maybe replace the gas stove with induction.
Or accept the higher CO2 levels. CO2 isn't personally dangerous like CO is. It's not ideal, but I think those warnings about loss of cognitive function are excessively alarmist.
I'd guess the author has a window that doesn't open completely or doesn't have windows on opposite sides of the building. In my experience, it's much harder to get good airflow if the path looks like a U or an L, instead of a straight line.
Check the ventilation system to see if it's working properly. It may need to be cleaned or replaced. Maybe have some plants in your home. Maybe replace the gas stove with induction.
Or accept the higher CO2 levels. CO2 isn't personally dangerous like CO is. It's not ideal, but I think those warnings about loss of cognitive function are excessively alarmist.