For a vehicle with a P0420 trouble code failing due to excessive NOx, what should be checked prior to replacing the catalytic converter?

The best choice in this scenario is to check the upstream and downstream oxygen sensor readings before replacing the catalytic converter. The P0420 trouble code indicates that the catalytic converter is not performing efficiently, but this condition can often be influenced by upstream components such as oxygen sensors, fuel mixture, and engine operation.

By examining the upstream and downstream oxygen sensor readings, you can determine if the sensors are operating correctly and whether they are providing accurate data to the engine control unit (ECU). If the upstream oxygen sensor is malfunctioning, it can cause poor fuel mixture and lead to high nitrogen oxides (NOx) emissions. Likewise, if the downstream sensor is not reflecting the proper operation of the catalytic converter, it might not be truly indicative of a converter failure, necessitating its replacement unnecessarily.

Conducting this check can help diagnose whether the problem lies with the sensors or the catalytic converter itself, ensuring that you address the root cause of the issue and not just the symptom. This step is crucial in avoiding potentially wasted labor and cost on parts that may not resolve the vehicle's emission issue.