What type of modifications can lead to emissions testing failures?

Aftermarket exhaust systems that bypass emissions controls can significantly impact a vehicle's emissions during testing, which is why they can lead to testing failures. These modifications often remove or alter components designed to reduce harmful emissions, such as catalytic converters or mufflers that have emission-reducing features. By bypassing these systems, the vehicle may emit higher levels of pollutants than allowed under state regulations, resulting in a failure during emissions testing.

In contrast, upgraded tires, advanced audio systems, and custom paint jobs do not affect the vehicle's emissions output in a way that would lead to testing failures. While these modifications might enhance performance, aesthetics, or comfort, they do not interfere with the vehicle's emissions control systems or contribute to higher emissions levels.