Carbon monoxide has an atomic weight of 28. This is less than that of Oxygen, 32.
It is equal to that of Nitrogen, the primary component of air.
Nitrogen and Oxygen make up 99% of what we call air.
In otherwords, Carbon monoxide is lighter than air in general, and neutrally buoyant relative to its primary component, Nitrogen.
So Carbon monoxide will tend to rise to the top of an enclosed space, not fall.
Given that exhaust fumes are hot, convection alone would carry it upward.
Putting vents at the bottom is the worst ventilation strategy when it comes to carbon monoxide because it is contrary to basic science.
I'm the ICC is considering mandating them.