have had three furnaces in last two weeks with odd readings and found issues with them, which would not have shown up otherwise.
Replaced a hx in an 80% Goodman. was third one going in, so checked it over pretty hard. The one I condemned looked like it had an obstruction it it that caused the sooting in hx.
put in new one and 7% O2 on it. took shutters off burners and a little better, but still not right. Called Goodman and found that there was a service up date on the propane conversion kit yrs ago (change in orifice size). Ordered the corrected kit to put in. (correct orifices by the nameplate BTW)
90% Rheem furnace w/ low O2, sooted in hx. hit 6000+ppm CO in flue gases.
90% Trane w/ hi O2. Had to take a number of parts off to find cracked parts leaking air into a transition fitting to inducer. New part ordered there as well.
w/o CA would have been back for sooted up hxs later, or worse.