Possible causes of P0420:
Faulty oxygen sensor
Faulty air-fuel sensor
Worn or internally failing catalytic converter (Most Common)
A leak in the exhaust system
Misfire (the root cause of converter failure)
Rich or Lean air-fuel ratio (the root cause of converter failure)
Many times this code is thought to be a faulty O2 sensor or A/F Sensor; while a possibility, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired along with P0420. Codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are misfire codes, and these codes require misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will go bad again. Also, make sure you don’t have codes P0174, P0171, P0172, or P0175 which can mean the engine is running rich or lean, which can burn out your catalytic converter. If these codes or any other codes are present, they should be addressed first.