What could be a reason for a vehicle to fail an emissions test?

A vehicle may fail an emissions test primarily due to faulty oxygen sensors or exhaust leaks. Oxygen sensors play a crucial role in monitoring the amount of oxygen in the exhaust gases and providing this information to the engine control unit for optimal fuel mixture adjustments. If these sensors are malfunctioning, they can lead to an incorrect air-fuel ratio, resulting in incomplete combustion. This can cause the vehicle to emit higher levels of pollutants than allowed, leading to a failed emissions test.

Furthermore, exhaust leaks can introduce outside air into the exhaust system, disrupting the readings that sensors provide about exhaust composition. This can also lead to improper fuel management and, consequently, increased emissions. Both of these issues can significantly impact a vehicle’s emissions performance and are direct contributors to failing an emissions test.