What can cause a vehicle to fail the emissions test?

The reason issues like malfunctioning O2 sensors, exhaust leaks, or improper fuel mixture can lead to a vehicle failing the emissions test is rooted in their direct impact on exhaust emissions. O2 sensors play a critical role in managing the air-fuel mixture in the engine; if they malfunction, they can lead to an overly rich or lean mixture, which can produce excessive emissions. Exhaust leaks can introduce unmetered air into the exhaust system or allow harmful pollutants to escape before they reach the catalytic converter, which is crucial for reducing harmful emissions. Additionally, an improper fuel mixture can result in incomplete combustion, generating more pollutants than the vehicle's emissions system can effectively manage. Together, these factors compromise the vehicle's ability to meet regulatory emissions standards.

In contrast, worn tires and brakes, outdated vehicle registration, or low battery voltage may affect vehicle performance or safety, but they do not directly influence the emissions output as critically as the components related to the emissions system do.