What component is checked for proper installation during an emissions inspection?

The gas cap is checked for proper installation during an emissions inspection because it plays a crucial role in maintaining the integrity of the vehicle's fuel system and preventing fuel vapor leaks. A properly sealed gas cap ensures that fuel vapors do not escape into the atmosphere, which can lead to increased emissions and contribute to air pollution. An improperly installed or damaged gas cap may trigger the vehicle's onboard diagnostic system to activate a warning light, indicating potential issues with the emissions control system.

Other components such as the engine oil cap, spark plugs, and air filter are not directly related to emissions control during the inspection process. The oil cap primarily keeps engine oil contained, spark plugs are vital for engine performance and ignition, and the air filter contributes to engine efficiency but does not directly impact the emissions test focusing on vapor containment and fuel evaporation.