A vehicle can get a P0420 engine fault code when its catalytic converter reaches a system efficiency that is below the vehicle's typical threshold. The driver of the vehicle can notice symptoms of the P0420 engine fault code by feeling if the car's exhaust has a reduced power output.
A variety of issues can cause a P0420 engine fault code, such as a fuel tank that was filled using leaded fuel rather than the required unleaded fuel, oil contamination, a cylinder misfire, high fuel pressure or a leaking fuel injector. Other causes for this engine fault code also include similar readings on the oxygen sensors located behind and in front of the converter, a malfunctioning engine coolant temperature, a retarded timing of the engine's spark, or a damaged catalytic converter.
To troubleshoot the P0420 engine fault code, check for leaks at the catalytic converter, at the engine manifold or in the vehicle's various pipes. The catalytic converter's oxygen sensors can also be diagnosed using a scope to see if it is operating properly. Inspect and replace the oxygen sensor, which is located at the heated end of the engine. In cases of serious damage, the whole catalytic converter might require replacement.