P0420 Code - Oxygen Sensor

[smokeydogvol]

Good afternoon friends

So my son's 2009 Cube throws a P0420 code, which is likely one of the O2 sensors, but how do you know for sure and how do you know which one? Thank you for your help!

[VStar650CL]

If the code is by itself then it's most likely a dead catalyst and not a sensor. The only things that can fool the ECM are air leaks around the rear O2 sensor or a rear O2 that's flatlined in the normal voltage range (0.3~0.6V). So check if the rear O2 is oscillating normally at a cruise with the cat lit, if it is then the cat is dying.

[smokeydogvol]

Yeah buddy, that is above my pay grade and I do not even know what language you are speaking lol Sorry! I'll re read what the code said and get back to you, but I do not have the tech or knowledge to know what you said to do.

I had the cat converter replaced just a few years ago.

[smokeydogvol]

Shows 2 things really...

P0420 Catalyst System Efficiency Below Threshold Bank 1
P1421 But nothing more than that...

I just do not believe the catalyst is bad as I just replaced it a few years ago.

[VStar650CL]

If the car has never been back to a dealer, the P1421 could be an artifact of a glitch in the ECM firmware. There's a bulletin for it that specifies an ECM reprogram, NTB10-089:
https://www.tsbsearch.com/Nissan/NTB10-089

If that isn't it, then the most likely cause is a dirty Throttle Body and a miscalibrated MAF, possibly along with a restricted air filter. Since the MAF readings are used in concert with the A/F sensor (front O2), and the A/F is in turn checked against the rear O2, it's possible you have a chain of events that's fooling the ECM. So I'd resolve the P1421 first and see if the P0420 stays away with it. Clean the TB and perform an IAVL.

If not, what kind of cat did you put on? I've seen bargain basement ones fail (and even come apart) in well under two years. One Sentra we worked on 5 or 6 years back inhaled a cheapo cat less than 10K miles after installation, completely wrecking the engine. It's also possible the welds have sprung a pinhole leak near the A/F or O2, check for that by having an assistant partially block the exhaust with a shop rag.

[smokeydogvol]

Thank you sir. You again are very knowledgeable and helpful.

[VStar650CL]