A P0420 engine error code warns that the engine control module, which is the computer that monitors and controls engine performance, has determined the catalyst system efficiency is below minimum factory threshold settings. Causes include improper air intake, faulty fuel injection, faulty ignition or timing, or a faulty exhaust system.
The catalyst system passes burnt fuel exhaust across catalyst plates, allowing potential pollutants to recombine into less-damaging chemicals before being expelled out the tailpipe. A P0420 code means the system isn't working properly, allowing pollutants to be expelled with the exhaust. The error can also rest with the engine control module itself, which in turn generates a faulty P0420 code.