What does a moderate increase in HC with some decrease in CO2 suggest?

A moderate increase in hydrocarbon (HC) emissions accompanied by a decrease in carbon dioxide (CO2) suggests that there may be unburned fuel in the combustion process, which can point towards an exhaust leak. When an exhaust leak occurs, the exhaust gases that exit the engine can mix with outside air, leading to a dilution of the exhaust gases. This dilution can potentially lower the CO2 levels as the proportion of unburned hydrocarbons increases, indicating that the combustion process was not fully efficient, hence resulting in elevated HC levels.

In a situation where timing is advanced or retarded, the combustion characteristics would change, but typically not in the pattern of increased HC with a decrease in CO2. Advanced timing usually leads to more complete combustion and potentially increased CO2 levels, while retarded timing tends to produce higher CO and HC emissions but would not typically correlate with CO2 reduction in this manner. An ignition misfire would lead to a significant increase in HC emissions but not usually combined with a decrease in CO2. Therefore, the presentation of HC and CO2 levels leads to the conclusion that an exhaust leak is the more plausible explanation.