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

Modifications to the factory emissions control equipment can significantly impact a vehicle's emissions performance and lead to a failure in emissions testing. Emissions control systems are designed to reduce pollutant output from the vehicle's engine, including components such as catalytic converters, oxygen sensors, and exhaust gas recirculation systems. If these components are altered, removed, or not functioning as intended, the vehicle may produce higher levels of harmful emissions than what is legally permissible.

Upgrades to the audio system, changes in tire size, and replacement of the vehicle's battery generally do not affect the emissions control systems or the efficiency of emission-related components. While these modifications might improve aesthetics or performance in other areas, they do not alter how the engine interacts with emissions regulations. Therefore, only modifications made to the emissions control equipment itself can directly lead to an emissions test failure.