What could cause some increase in HC and CO, along with a decrease in CO2 and O2?

The scenario described involves an increase in hydrocarbon (HC) and carbon monoxide (CO) emissions, along with a decrease in carbon dioxide (CO2) and oxygen (O2) levels. This pattern indicates incomplete combustion occurring in the engine.

When there is general engine wear, it can contribute to poor sealing of the combustion chambers, leading to reduced compression and inefficient fuel burning. As the engine components wear out, factors such as increased tolerances and degraded piston rings can result in inadequate mixing of air and fuel and inefficient combustion. This inefficiency is represented by higher HC and CO emissions as unburned or partially burned fuel is expelled from the exhaust.

Additionally, a reduction in both CO2 and O2 can occur due to an increase in combustion byproducts (like CO and HC) as well as a result of less air being effectively mixed with the fuel, creating an imbalance in the combustion process. Therefore, the wear and tear of engine internals can lead to these specific changes in emission levels.

In contrast, air injection failure primarily affects emissions by increasing oxygen content in the exhaust, which would not lead to the specified decrease in O2. Similarly, while EGR leaking can cause increased HC and CO, it usually does not produce a