okay i know that there is alot of questions about emissions and i tried the search feature and there was a shitload on emissions, but as i read threw them everybody was saying that CO is a result of too rich of a mixture. but they say that HC is also a result of too rich of a mixture. my brother-in-law took his car in to emissions and passed the HC but failed the CO by .25. my question is if they are both as a result of the air-fuel ratio being too rich how can he pass one but fail the other? if someone can tell me what the diffrence between these two is it would really help me out. THANX.

 

CO is an incompletely-burned hydrocarbon, depending on who your chemistry teacher is...

Generally, CO & HC will both go up when you run rich. I don't know about a magic way to predict which one will fail you first.

NOx comes from too-high combustion temperatures. It'll go down when you run rich, because rich will lower the temperatures. NOx will go up if you advance your spark timing, because that will increase combustion temperature.

Obviously, the trick is getting em all low enough at the same time...