If you think the CO is coming from this unit, it really needs to have a combustion analysis done, because it is producing way too much CO.
Here is an explanation by Jim Davis about how much CO the unit has to produce, to get any CO inside the structure.
A 100,000 btu furnace uses 25 cfm of air for complete combustion. Furnace blower in heating move 1300 cfm on draft induced and 1500 cfm on condensing.
If you mixed 100% of the flue gasses with 1500 cfm how much CO would you have to be making to get a reading of 10ppm?
25cfm is .016% of the mixture, therefore the furnace would have to be making over 600ppm which exceeds the 400ppm "air free" they are allowed.
The furnace would have to be red-tagged without a hole or crack.
That is dumping 100% of the flue gasses into the airstream. A crack may under certain conditions may leak say 2% of the 25 cfm or say .5 cfm. .5 cfm is .003% of 1500ppm.
So to read 10ppm of CO in the airstream, the furnace would have to be making over 3000ppm of CO.
Even without the blower running the volume of air in the plenum versus the volume of air from a leak would be still be quite diluted.
Bottom line: To get measureable CO in a space an appliance,or vehicle, has to be making seriously high levels of CO, or it has to have been made over long periods of run time.
Instead of learning the tricks of the trade, learn the trade.