The furnace that was in this house when I bought it must have been installed by an amateur. The ducting he installed was not large enough diameter, nor were the registers large enough, to provide the proper air flow. This year the lack of flow caused the heat exchangers to overheat, which in turn caused them to buckle, allowing the exhaust gasses to get into the ducting. I don't know what the CO level was, but it sure did stink! And there I was with COPD breathing noxious gasses. Yuk.
I got a new furnace and ducting. The new ducting is way bigger than the old. Expensive, but what could I do?
And they put in a CO detector. Rather like closing the barn door after... Oh,well.