If a vehicle fails a smog check with high CO emissions and a P0172 trouble code, what is the best first step in diagnosing the issue?

The best first step in diagnosing the issue when a vehicle fails a smog check due to high carbon monoxide (CO) emissions and a P0172 trouble code is to check the oxygen sensor readings and fuel trims.

The P0172 trouble code indicates that the engine control module (ECM) has detected a rich fuel condition, meaning the air-fuel mixture is too rich (too much fuel compared to air). High CO emissions are often associated with a rich condition, as incomplete combustion of the excess fuel can lead to an increase in carbon monoxide levels. By assessing the oxygen sensor readings, you can determine how well the oxygen sensors are functioning and if they are sending accurate data to the ECM regarding the air-fuel mixture.

Additionally, examining fuel trims—both short-term and long-term—will provide insight into how the engine is adjusting the fuel delivery based on real-time conditions. This step helps in pinpointing whether the engine is compensating correctly or if there is a fundamental issue leading to the rich condition, such as a faulty sensor or incorrect fuel pressure.

While checking other components like the fuel injectors, vacuum leaks, or the catalytic converter is important in the overall diagnostic process, focusing on the oxygen sensors and fuel trims provides a direct