Q: p4020 code is my o2 sensor bad?

Hi there:

The OBD-II error code P0420 generally indicates a Catalyst System Efficiency is below threshold (Bank 1). The upstream oxygen sensor is located in front of the catalytic converter. If the upstream oxygen sensor is working properly, its readings should fluctuate when the car is warm and when in closed loop. The downstream oxygen sensor is located behind the catalytic converter. If the downstream oxygen sensor is working properly, its readings should remain steady. In general, when this error code shows up, it means one of the two O2 sensors is either faulty, the wiring is faulty or the catalytic converter itself is damaged. To be certain, you might want to have a professional mechanic complete a check engine light inspection which will allow them to pull all codes stored in the ECU to determine which of the sensors might be faulty.