Which of the following is a reason for a vehicle to fail a smog check?

A vehicle may fail a smog check primarily due to excess emissions beyond the allowable limits set by environmental regulations. Each vehicle is required to meet specific standards for pollutants such as hydrocarbons, carbon monoxide, and nitrogen oxides. When a vehicle emits higher levels of these pollutants than permitted, it indicates that the emissions control systems are not functioning properly, or that there may be other mechanical issues contributing to higher emissions.

In contrast, compliance with emission standards is a sign that the vehicle is operating correctly, while routine maintenance suggests that the owner is proactive about keeping the vehicle in good condition. Furthermore, verification of the vehicle's condition typically relates to confirming that all parts are functioning properly and maintenance is up to date, which can prevent failures. Thus, the key factor for a smog check failure is indeed the presence of excess emissions surpassing the allowable limits.