The most probable cause is combustion of a rich mixture of fuel.
to much fuel. gm jim Fuel to air mixture too rich or choke stuck on.
A rich condition.
bad or misreading MAP or MAF sensors, bad ECT sensor, bad TPS sensor, stuck open fuel injector, or problem with fuel system, faulty parameters set in ECU or bad ECU.
High CO is a rich air/fuel mixture. Nothing else.
It is partially clogged, cylinder miss, or the fuel/air mixture is too rich.
Usually when a Catalic Converter becomes unusable, it is usually caused by a enriched fuel system. The rich fuel mixture causes the internal catalyst to become they usually generally melted down inside. This excessive heat is built up when the engine is running in a "too rich fuel mixture". After the converter has been replaced, the automobile should be taken to a repair facility, and have them connect their electronic control monitoring equipment to your on board computer, so the reason for the enriched fuel mixture can be determined and repaired.
mixture is too rich, is this carb. or injection? Have it checked. Some of this discoloration is normal - caused by carbon monoxide.
You are running too rich. the mixture of fuel and air has too much fuel in it,
A leaner fuel mixture burns hotter than a rich mixture. so a leaner mixture will be more prone to detonation than a richer mixture.
Too rich of a fuel/air mixture, improper oil/fuel mixture, improper combustion, mechanical malfunction... all possible reasons.