What might cause a vehicle to fail an emissions inspection?

A vehicle may fail an emissions inspection if it has a malfunctioning or absent catalytic converter. The catalytic converter is a critical component of a vehicle's exhaust system, designed to reduce harmful emissions by converting toxic gases like carbon monoxide, hydrocarbons, and nitrogen oxides into less harmful substances before they are released into the atmosphere. When the catalytic converter is not functioning properly or is missing entirely, the vehicle will emit higher levels of these pollutants, which is a primary concern for regulations aimed at protecting air quality.

The other options do not directly impact the emissions output. A fully functional catalytic converter ensures that emissions are reduced effectively, while good tire conditions and a clean windshield are unrelated to the vehicle's emissions performance. Therefore, having an intact and functioning catalytic converter is essential for passing emissions inspections, making the presence of a malfunctioning or absent one a significant reason for failure.