Which of the following could be a reason for a vehicle failing a Smog Check?

Exceeding the allowed emissions limits is a significant reason why a vehicle could fail a Smog Check. The primary purpose of a Smog Check is to ensure that vehicles meet the environmental standards set by legislation. Each vehicle has specific emissions limits established by the California Air Resources Board (CARB) that must not be exceeded. When a vehicle emits pollutants beyond these established thresholds, it indicates that the engine or emissions control system may not be functioning correctly, potentially harming air quality and public health.

In contrast, valid registration, recent routine maintenance, and passing previous inspections all contribute positively to a vehicle's overall condition and compliance with regulations. However, these factors do not directly affect the emissions output at the time of inspection. Valid registration is a legal requirement but does not gauge emissions levels; routine maintenance typically aims to improve vehicle performance but may not necessarily address emissions issues, and past passing inspections indicate prior compliance but do not guarantee current performance. Therefore, only the failure to meet emissions limits directly results in a Smog Check failure.