P0420 Fault on 2001 Nissan Sentra: Oxygen Sensor Replacement and Catalyst Efficiency Issues
I recently replaced the bank 2, sensor 2 oxygen sensor as it triggered a fault code and was not functioning properly. After the replacement, the check engine light turned off, but came back on after driving approximately 10 miles. A diagnostic scan revealed code P0420, which indicates 'catalyst system efficiency below threshold'. The technician at AutoZone advised using higher octane fuel, but I'm concerned whether replacing just one oxygen sensor might have caused this issue. Since there are four oxygen sensors in total (two per bank), should I consider replacing all of them now? I want to ensure the catalytic converter is functioning correctly and avoid further engine performance issues.
Check the voltage reading of the oxygen sensor located before the catalytic converter. It should switch between approximately 100 mV and 900 mV. Then, check the voltage of the sensor after the catalytic converter—this should read around 200 mV. If both sensors show similar readings (mirroring each other), it suggests the catalytic converter is likely faulty. Additionally, pay attention to any unusual smells such as rotten eggs or noises like a rattle during acceleration, which may indicate exhaust system problems near the catalytic converter area.