What emissions increase is likely due to vehicle overheating?

When a vehicle overheats, it primarily affects the combustion process within the engine, which can lead to an increase in hydrocarbon (HC) emissions. High temperatures during combustion can cause incomplete burning of the fuel, resulting in unburned hydrocarbons being released into the exhaust. This is because the high temperature can lead to the formation of fuel-rich conditions that promote the production of HC emissions as fuel and air might not mix properly due to thermal expansion and other factors related to overheating.

While carbon monoxide (CO) can also increase under certain conditions, including when there is incomplete combustion, it is the hydrocarbons that typically show a more significant increase specifically tied to overheating issues since they are unburned fuel escaping into the exhaust system. Additionally, oxygen levels in the exhaust may not significantly drop in an overheating scenario, as the primary problem revolves around the chemical reactions occurring in the combustion chamber and not necessarily a lack of oxygen.

Thus, the increase in HC due to overheating is the most visible indicator of combustion efficiency issues related to this thermal condition, making it the most appropriate choice in this context. The other emissions do not directly correlate as strongly with vehicle overheating.