# fixing P0420 on 2006 Nissan Altima



## juluribk (Jan 8, 2017)

Hi!

I have a 2006 Nissan Altima 2.5 L and has service engine light with P0420 error code (Catalyst System Efficiency Below Threshold (Bank 1)).

The engine was rebuilt last year by a mechanic because the timing chain tensioner had gone bad and he suspected that engine was burning oil. 

I am trying to understand if the current error code is because of a bad catalytic converter or because of oxygen sensors gone bad. I bought the car at 80k miles and now has 135 K miles, I have never changed the oxygen sensors.

I ran the scan tool and I got the sensors readings after I warmed the car and maintained the speed at ~2500 rpm. 

If I understand correctly, oxygen sensor 1 is the sensor before catalytic converter and O2 sensor 2 is the sensor after catalytic converter.

Scan data shows that O2 sensor 1 voltage is not at all oscillating, I understand that it should be oscillating back and forth representing lean and rich conditions of exhaust. Does it mean my O2 sensor 1 is bad? 

Scan data shows that O2 sensor 2 voltage is slowly increasing its voltage and then starts to oscillate after some time. I thought it should remain constant. Does it mean O2 sensor 2 is also bad or is it good and it is behaving weird because of bad O2 sensor 1?

Appreciate your suggestions. I cannot post any data (screenshots) here in this thread because it is my first post.


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## smj999smj (Jan 1, 2006)

To answer your questions: no, not necessarily and no. The upstream sensor, although it looks like an oxygen sensor, is actually an air/fuel ratio sensor. The rear sensor, behind the catalyst, is an oxygen sensor. Air/fuel ratio sensors operate a little differently than oxygen sensors and it is difficult to determine if they are working properly by just viewing datastream info. Air/fuel ratio sensors don't show as much oscillation compared to an oxygen sensor. The best way to tell if you have a bad air/fuel ratio sensor is when the ECM tells you it's bad by setting a code for it and triggering the check engine light. If you are burning oil, this will quickly ruin a catalytic converter. The most common cause for a P0420/P0430 code where another engine management code is not stored (i.e. MAS malfunction, oxygen sensor, cam position fault, etc.) is an inefficient catalytic converter.


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## quadraria10 (Jul 6, 2010)

In my books if you are going to keep it, its worth changing the o2 sensors as they are past due anyway. The worst case scenario sees you replacing the front cat manifold, and if you did it would be nuts to reuse degraded 10 year old o2 sensors. If the cat is still good, it just may fix your problem.


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