The emissions part of your state inspection failed What is the most common cause?

The most common cause of an emissions inspection failure is a faulty or malfunctioning oxygen sensor. The oxygen sensor is responsible for measuring the amount of oxygen in the exhaust gases, which is used to calculate the air-fuel ratio. If the oxygen sensor is not working properly, the engine may be running too rich or too lean, which can cause emissions levels to exceed legal limits. Other possible causes of emissions inspection failure include a faulty catalytic converter, a damaged exhaust system, or a malfunctioning engine management system.