What might indicate that a vehicle is in need of a smog check?

A vehicle may require a smog check for several reasons, which is why the choice indicating that all the provided options are valid signals is correct.

When there is a decrease in fuel efficiency, it often points to an underlying issue that may increase emissions, suggesting the need for inspection to ensure compliance with environmental standards. Similarly, an unusual sound from the engine can indicate mechanical problems that could affect how the vehicle runs and its emissions levels. The check engine light being illuminated is a critical indicator, as it directly signals that the vehicle's onboard diagnostics have detected a failure in the emissions control system or related components. Each of these factors is associated with potential issues that can lead to increased emissions, necessitating a smog check to identify and rectify the problems. Thus, considering all these factors collectively emphasizes the importance of regular inspections to maintain the vehicle's environmental compliance and performance.