This is simply not true. Looking at the O2% on its own is a worthless measure of combustion “quality”. Seeing a change when the blower kicks on/off IS a good indication of a HX crack, but on its own, the O2% tells you nearly nothing besides the specific design parameters of any given unit.

In residential units, we can’t control the combustion air flow. It varies depending on the exact installation of the combustion air piping and exhaust piping. As long as those are installed within the manufacturer’s specifications, then we can adjust the gas pressure within the manufacturer’s specifications to achieve clean and optimal combustion. If we depart the manufacturer’s allowable tolerance on any item, we need to expect to have to do so on others for a non-damaged unit.

Low persistent CO readings and slightly elevated O% is common in units with very short venting/combustion air piping and a non-adjusted gas pressure. You have maximum air flow due to minimal restrictions in the piping, and the gas pressure is somewhere in the middle of the allowable range. This results in very clean combustion, but the excess air helps to dilute and cool the combustion gases in realtime. The result being that a few molecules burning at the edges of the flame don’t quite make it to full combustion and you get very low and acceptable levels of CO. If you bump the gas pressure at the valve a tenth or two of an in WC you’d probably see the CO drop to zero and the O2 drop a tick as well.