Which condition would cause some increase in HC, a large increase in CO, and a moderate decrease in CO2?

The situation described, where there is an increase in hydrocarbons (HC), a large increase in carbon monoxide (CO), and a moderate decrease in carbon dioxide (CO2), is indicative of insufficient combustion occurring in the engine. Air injection systems are designed to inject additional air into the exhaust stream, which helps in the oxidation of unburned hydrocarbons and CO, converting them into CO2 and water.

When air injection fails, there isn't enough oxygen in the exhaust for complete combustion to occur. As a result, unburned hydrocarbons increase, leading to higher HC readings. The lack of sufficient oxygen also results in a higher concentration of CO, which is a byproduct of incomplete combustion, since more fuel remains unburned. Concurrently, as CO2 is a product of complete combustion, the failure of air injection would lead to a lower amount of CO2 being produced, resulting in a noticeable decrease in its levels.

In contrast, conditions like leaking EGR systems or low compression would have different effects on emissions profiles, and general engine wear would present a broader range of symptoms rather than the specific emissions characteristics mentioned. Therefore, the failure of the air injection system fits best with the described changes in emissions.