The error code P0420 indicates that the catalytic converter's efficiency is below the threshold for the bank 1 side of the engine. This issue can manifest as a decrease in fuel efficiency, a noticeable drop in engine performance, or the vehicle failing an emissions test. The severity of the symptoms can vary, but the check engine light will typically illuminate as a warning sign. Drivers may also notice a sulfur or rotten egg smell from the exhaust, indicating incomplete combustion or that the catalytic converter is not processing exhaust gases properly.

The root cause of a P0420 code can be traced back to several factors, but the primary culprit is often the catalytic converter itself failing to catalyze the exhaust gases efficiently. Other potential causes include oxygen sensors not functioning correctly, exhaust leaks, or engine misfires leading to excessive unburned fuel entering the catalytic converter. The catalytic converter's role is to convert harmful pollutants into less harmful emissions before they exit the exhaust system. If left unaddressed, a faulty catalytic converter can lead to increased emissions, reduced fuel economy, and potential damage to other components, including the oxygen sensors and the engine due to back pressure.