- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module (ECM)

As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

Very common problem with Nissan vehicles. There are Nissan factory service bulletins for the following Nissan models:

2000 Nissan Maxima
2001 Nissan Maxima
2000-2001 Nissan Maxima Factory Service Bulletin OBDII Code P0420

2001 Nissan Frontier
2001 Nissan Xterra
2001 Nissan Frontier-Xterra Factory Service Bulletin OBDII Code P0420

2004 Nissan Altima
2005 Nissan Altima
2004-2005 Nissan Altima Factory Service Bulletin OBDII Code P0420

2004 Nissan Altima
2005 Nissan Altima
2006 Nissan Altima
2004-2006 Nissan Altima Factory Service Bulletin OBDII Code P0420

2005 Nissan Frontier
2006 Nissan Frontier
2005-2006 Nissan Frontier Factory Service Bulletin OBDII Code P0420

- Three way catalytic converter Bank 1 does not operate properly.
- Three way catalyst converter Bank 1 does not have enough oxygen storage capacity.

- Engine Light ON (or Service Engine Soon Warning Light)

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).
A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase.
When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.