A vehicle fails smog due to excessive CO emissions. Which of the following is the MOST likely cause?

When a vehicle fails smog due to excessive carbon monoxide (CO) emissions, the most likely cause is related to the engine management system that controls air-fuel mixture, and a malfunctioning mass air flow (MAF) sensor can lead to such a failure.

The MAF sensor is responsible for measuring the amount of air entering the engine, which is crucial for the engine control unit (ECU) to calculate the correct amount of fuel to inject. If the MAF sensor is faulty or providing incorrect readings, it can result in an overly rich fuel mixture—too much fuel relative to the amount of air. This rich mixture can lead to incomplete combustion, producing higher CO emissions as a byproduct.

While other components like the throttle body, fuel pump, and coolant temperature sensor each play important roles in engine performance and emissions, they are less directly responsible for excessive CO output in the way that a MAF sensor is. For example, a malfunctioning throttle body may affect airflow but is less likely to contribute to an overly rich fuel condition. A failing fuel pump could lead to poor engine performance, but typically does not result in excessive CO unless it is causing issues with fuel delivery. Similarly, a coolant temperature sensor that is malfunctioning can affect the