emission failure on 1996 Saab 900

What is the most common problem when the engine is running too rich?

Asked by for the 1996 Saab 900
Too rich can be due to so many things. An engine out of tune, too high a fuel pressure (bad fuel pressure regulator), fuel injectors that are not atomizing fuel spray properly, ignition misfire due to weak spark or bad spark plugs, restricted air filter, or an emission test may "show high CO" if the catalytic convertor is not neutralizing the harmful exhaust gasses emitted from the engine/