During an emissions test, high hydrocarbon levels with normal oxygen sensor readings may indicate what issue?

High hydrocarbon levels with normal oxygen sensor readings during an emissions test typically suggest that there is an issue with the fuel delivery system, specifically with the fuel injectors. When fuel injectors malfunction, they may become clogged or fail to dispense the correct amount of fuel into the combustion chamber. This can lead to incomplete combustion, resulting in higher hydrocarbon emissions, which are a byproduct of unburned fuel.

While normal readings from the oxygen sensor indicate that the air-fuel mixture is being monitored correctly and that the oxygen in the exhaust system is at expected levels, the presence of high hydrocarbons indicates that not all the fuel is being burned properly. This scenario strongly correlates with injector problems, as they play a crucial role in maintaining the proper air-fuel mixture necessary for complete combustion.

Other options, such as faulty spark plugs, clogged air filters, and excessive exhaust back pressure, can also lead to emissions issues, but they typically would show other signs in the oxygen readings or affect the combustion process in different ways.