Which of the following is a common cause for a vehicle to fail an emissions test?

A vehicle often fails an emissions test primarily due to issues with the emissions control system. The catalytic converter and oxygen (O2) sensors play crucial roles in reducing pollutants produced by the engine. The catalytic converter converts harmful gases like carbon monoxide, hydrocarbons, and nitrogen oxides into less harmful emissions. If the catalytic converter is faulty, it cannot effectively reduce these pollutants, leading to higher emissions that exceed acceptable levels.

Similarly, malfunctioning O2 sensors can provide incorrect readings to the engine control unit, causing an improper air-fuel mixture. This can result in either an overly rich or lean fuel mixture, both of which can increase harmful emissions. Therefore, issues with these components directly impact the vehicle's ability to pass an emissions test, making them the most relevant cause of failure in the given scenario.

In contrast, while worn brake pads and excessive tire wear can affect vehicle safety and handling, they do not influence the vehicle’s emissions performance. Consequently, they are not common causes of failure in emissions testing.