What does high carbon monoxide in an engine typically signify?

High carbon monoxide (CO) levels in an engine's emissions typically signify that the engine is running rich. A rich fuel mixture means that there is more fuel in the combustion chamber relative to the amount of air. This excess fuel does not combust completely during the combustion process, leading to higher levels of carbon monoxide being emitted.

When an engine runs rich, the combustion process is inefficient. Instead of producing more power, the excess fuel leads to incomplete burning, resulting in CO as a byproduct. This is in contrast to an efficient combustion process, which produces more carbon dioxide (CO2) instead of CO.

In summary, detecting high levels of carbon monoxide in an engine is a clear indication of a rich fuel mixture, highlighting issues with the air-fuel ratio that can lead to suboptimal engine performance and increased emissions.