2003 nIssan maxima P0420

I have a 2003 nissan maxima with 194k on it and i just changed the fron cat conv and O2 sensor and now i get a P0420 code which has something to do with the efficency of the cat on bank 1, what does that mean? and what do i need to change to clear the code?

Why did you change the Cat?? Were you getting a CEL/code before you changed the Cat?

The front catalytic converter is called the pre-catalyst. The catalytic converter located under the vehicle is called the main catalyst. The code indicates that there’s an effeciency problem with the main catalytic converter. And with a 194k on the main catalytic converter, it should come as no suprise that the efficiency of the main catalytic converter has fallen off.


I had a code in there before and if i’m not mistaken it was either p0430 or P0420 but def. not both and the dealer advised to change the front cat and O2 sensor.

So does that mean the sensors are bad too or just the cat? and can both cats go bad t once?

The catalytic converter’s job is to clean up your exhaust. Your O2 sensors measure the contents of your exhaust. There are O2 sensors before the catalytic converter (upstream or “sensor 1”) and sensors after the cat (downstream or “sensor 2”). Bank 1 is the row of cylinders with cylinder one on it. Four cylinder cars have only one “bank.” Six & 8s have 2 banks (though not always 2 converters - it depends on the exhaust system design).

The O2 sensors generate an electrical signal that bounces up and down depending on exhaust contents. The upstream sensor normally bounces around a lot. The downstream sensor should bounce around a lot less because the converter cleans up the exhaust. If the computer sees readings from the downstream sensor that look too much like the ones from the upstream sensor it will set the P0420.

ONE cause for this kind of thing is a converter that isn’t doing is job well enough - leaving the exhaust too dirty.

But there are other causes. If you want anyone to tell you anything of substance you have to go back to the beginning and provide the full story. It seems to have started with a check engine light and a trip to the dealer. What codes were there? What did the dealer do to actually find out the cause of the code? Did they check for exhaust leaks? Look at the actual O2 sensor signals? Actual exhaust gas analysis? Or what? Then, who actually did the work? You? Double check what was done. Was it a precat that was replaced? Or the actual cat? IF the actual cause is a bad cat AND IF you only replaced the precat, then as Tester said, its no surprise that you have the P0420. Unless the initial code was P0430 - which is the same as P0420 but for bank 2 instead of bank 1.