What constitutes a 'major modification' that requires an updated Smog Check?

An updated Smog Check is required for 'major modifications' that affect the emissions system of a vehicle. This includes significant changes such as swapping out engines, which can directly impact how the vehicle generates and controls emissions. Modifying the engine can result in alterations to the way exhaust gases are produced and processed, thus necessitating a reassessment of the vehicle's compliance with environmental regulations.

Looking at the other options, changes that only impact aesthetic features do not influence emissions and therefore do not require a Smog Check. Similarly, minor repairs to the interior or upgrades to the sound system, while they may enhance the vehicle's comfort or entertainment value, do not have a direct connection to emissions control and thus do not necessitate an updated inspection. Understanding the specific modifications that affect emissions helps ensure that vehicles remain compliant with California's stringent air quality standards.