Mercedes Benz

P0420 is a diagnostic trouble code (DTC) that indicates the catalyst system's efficiency is below the threshold on bank 1 of the engine. The "bank 1" refers to the side of the engine where cylinder 1 is located. In vehicles with inline engines, there is only one bank, so the code pertains to that bank. The catalytic converter is an emissions control device located in the exhaust system. Its primary function is to convert harmful pollutants in the exhaust gases, such as carbon monoxide (CO) and hydrocarbons (HC), into less harmful compounds like carbon dioxide (CO2) and water vapor. The efficiency of the catalytic converter is monitored by the vehicle's engine control module (ECM) through the use of oxygen (O2) sensors, which are located before and after the catalytic converter. When the ECM detects that the downstream (post-catalytic) O2 sensor's readings are too similar to the upstream (pre-catalytic) O2 sensor's readings, it indicates that the cata...