Hi Kate,

Was the offending pollutant "CO" or "CO2"? We give off CO2 (carbon dioxide)when
we exhale and trees breath it in. IF it's CO(carbon Monixide) then your dead
rich meaning too much fuel is being injected into the combustion process. It
could be as simple as misadjusting the air bypass screw  on the air flow meter
or more complicated like excessive fuel pressure or temp sensors out of specs.
If you drove across country excessively rich, your catalytic converter could
now be damaged as a rich mixture overheats the cat. A non-functioning cat would
only make your E check failure more noticeable. First you should check the basic
tune up items and rethink ANYTHING that was done previously to correct other
problems as a rich engine can conceal unmetered air leaks, injector spray problems,
etc.

Regards,     John