What emission imbalance would most likely suggest a malfunctioning sensor?

The scenario of low carbon monoxide (CO) combined with high hydrocarbon (HC) emissions typically indicates an issue with the combustion process within the engine. In a properly functioning engine, you would expect CO levels to increase alongside HC emissions if there were incomplete combustion due to an excessively rich fuel mixture or other factors. However, low CO levels with high HC levels suggest that the fuel is not being burned efficiently, potentially due to a malfunctioning sensor, such as the oxygen sensor or mass air flow sensor, which can incorrectly report the air-fuel mixture, leading to improper fuel delivery and combustion conditions.

In this context, you can infer that a malfunctioning sensor might be misjudging the amount of air or fuel, resulting in incomplete combustion and an excess of unburned hydrocarbons, hence the elevated HC readings. The other situations do not typically suggest sensor malfunctions in the same way—rather they indicate expected behavior in various combustion scenarios. For example, normal combustion measurements would not imply malfunction, and other combinations like high CO2 with low O2 can point to normal combustion under certain conditions.