Interesting...
I'd expect high CO to translate to a rich mixture, since every C atom
yearns to find two O atoms (lesser energy state) once the reaction has
started, and if you get fewer O atoms than twice the number of C
atoms, you'd end up with some CO molecules in the exhaust.

By the same logic, high HC should beone of the two cases:
 a)VERY rich mixture, so some stuff gets thrown out unburned
 b)some cylinder(s) misfiring intermittently, so that even if the
   mixture is lean, you still get some unburned HCs in the exhaust

Do not take these arguments to be facts; I'm no chemist -- I'm just
thinking out loud...
Cetin

David Schwarze writes:
[snip...]
 > High HC (HydroCarbons) is a sign of running rich.  DO ask the experts,
 > cause this guy is wrong.  CO has little to do with the rich/lean issue,
 > although I have been told that hi CO can indicate a lean condition.