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 through a variety of symptoms, including reduced fuel efficiency, a noticeable decrease in engine performance, and sometimes an unusual smell from the exhaust. The frequency of these symptoms can vary, but they often become more noticeable during steady driving conditions or under heavy engine load. This problem directly affects the vehicle's ability to properly manage exhaust emissions, potentially leading to failed emissions tests in regions where such testing is mandatory.

The root cause of a P0420 error code can be attributed to several factors, but most commonly, it points to a failing catalytic converter. The catalytic converter is designed to reduce harmful emissions by converting exhaust gases into less harmful substances. Over time, the converter can become clogged, contaminated, or physically damaged, leading to decreased efficiency. Other potential causes include faulty oxygen sensors or exhaust leaks affecting the sensors' readings. Ignoring this issue can lead to significantly increased emissions, reduced fuel economy, and potentially more severe engine damage if unaddressed.