What kind of vehicle modifications might lead to emissions test failure?

Removing or altering the catalytic converter is a significant modification that can lead to emissions test failure due to the essential role the catalytic converter plays in the vehicle's emissions control system. This component is designed to reduce harmful emissions by converting toxic gases produced during combustion into less harmful substances before they exit the vehicle's exhaust system. When this component is removed or modified, it can drastically increase the amount of harmful pollutants that the vehicle emits, which is why such actions can result in a failure during an emissions test.

In contrast, upgrading the tires, installing a new sound system, or changing the color of the vehicle does not typically affect the emissions output of the vehicle. These modifications might improve performance, comfort, or aesthetics but don't have a direct impact on the vehicle's emissions control systems. Thus, they are less likely to contribute to a failure in an emissions test.