The P0420 code is typically triggered by the downstream oxygen sensor, also known as the post-catalytic sensor. This sensor is located after the catalytic converter in the exhaust system. More information: The downstream oxygen sensor monitors the level of oxygen in the exhaust gases after they've been processed by the catalytic converter. Its main function is to provide feedback to the car's engine control unit about the efficiency of the converter. If the converter isn't functioning properly, it can lead to abnormal readings by the downstream O2 sensor, triggering a P0420 code. However, remember that the P0420 code doesn't necessarily mean the sensor itself is faulty - it could be indicating another issue, like a malfunctioning catalytic converter, an exhaust leak, or a poor fuel mixture.