What does it indicate if hydrocarbons in an engine are high while carbon monoxide levels are low?

When hydrocarbons are high while carbon monoxide levels are low, it indicates that the engine is operating very lean. A lean mixture means that there is a higher proportion of air relative to fuel in the combustion process. This condition can lead to incomplete combustion. The excess air does not allow for optimal conditions for complete fuel burning, resulting in unburned hydrocarbons being emitted into the exhaust. On the other hand, low levels of carbon monoxide suggest that there is sufficient oxygen for combustion, leading to the conclusion that the engine continues to produce hydrocarbons without creating carbon monoxide from incomplete combustion—characteristic of a lean running condition.

Other conditions, such as running rich, would generally lead to higher levels of carbon monoxide due to incomplete combustion resulting from an excess of fuel. Misfiring tends to produce a chaotic combustion process that would likely increase both hydrocarbons and carbon monoxide levels. Receiving too much air alone does not describe the condition accurately, as it's the poor fuel-to-air ratio that is key in this situation.