Well after digging out my old text books I found my notes on five gas theroy and here goes. a very high CO and very low O2,NOx with a mid low CO2,and HC indicates an A/F of about 10:1 you are to LEAN!!! the trick to controlling CO emissions during combustion is to make sure you have the right amount of O2 and fuel in the mix you want enough O2 to form CO2 but not so much that you develop a lean misfire A lean misfire decreases performance and increases HC emissions When the mix is rich the engine produces a lot of CO As the mix passes the stoichiometric (14.7:1) ideal and goes lean When the mix is lean CO levels out and the curve no longer indicates anything CO is only a useful indicator of a rich mix. sorry for the long response but this is a very involved subject Keith