The error code P0420 indicates that the vehicle's onboard diagnostics (OBD) system has detected that the catalytic converter is not operating as efficiently as it should. This issue can manifest through a variety of symptoms such as reduced fuel efficiency, a noticeable decrease in engine performance, and possibly an increase in exhaust emissions. The severity of the issue can vary, but it often results in the vehicle failing emissions tests, which can prevent registration in many areas. Drivers might also notice a sulfur or rotten egg smell from the exhaust, indicating incomplete combustion or that the catalytic converter is struggling to process exhaust gases properly.

The root cause of a P0420 code can stem from several issues, but it primarily points to a malfunctioning catalytic converter. However, it could also be due to faulty oxygen sensors, exhaust leaks, or issues with the fuel system that lead to improper air-fuel mixture. The catalytic converter's role is to convert harmful pollutants in the exhaust gas into less harmful emissions. When it fails, it can no longer efficiently reduce exhaust emissions, leading to increased pollution and potential damage to other components, such as the oxygen sensors. Ignoring this issue can lead to more significant problems, including a complete failure of the catalytic converter, which can be quite costly to replace.