Hey guys, My registration expires this month, and I can't get it without an inspection which I can't pass because I have a check engine light on. It's giving me code P0420 Catalyst system below efficiency bank 1. I have an aftermarket intake which I think is the root of all my check engine codes that I want to get replaced back to a stock air box and not have to worry about it. It's $800 at the dealership and I can't find one for my car (2001 Nissan Maxima SE) anywhere else. I tried buying one off of a member on the forums and got screwed out of my money, and most importantly out of a stock air box. Besides that, I searched on the forums and found out that the part that would be the cause of this code is the front exhaust tube. I called Nissan to find out how much the part costs and they quoted me ~$671. I doubt mine is damaged, but I don't know what to do. I am going to jack my car up, and clean all the sensors and make sure they are all working right, but if that doesn't work it looks like I'm going to be looking for that front exhaust tube to get my check engine light fixed. Do you guys have any other ideas or am I pretty much on the right track?

 

Do some research on the P0420 code as many, many have the code.
The code is for a cat efficiency problem bank 1. The O2 sensor reading is
elevated because of the oxygen in the system due to the bad cat, or what
might end up being be a bad O2 sensor.
I'm going to cut to the chase and say, first replace the downstream sensor
bank 1.
If the code returns after the new sensor is installed, just use 2 spark plug
non-foulers and be done with the problem. There is a lot of info on the how to
on this forum or the net. Just a word of caution, using the non-foulers is illegal,
but the alternative is big $$$$$$.

 

Code 0420 blaze it up

 

Not your intake. Start by replacing the O2 sensor someone else mentioned. The non fouler a are not legal but kind of hard to spot unless the person is doing a detailed visual inspection.

 

Use the O2 extender/non-fouler. Your intake is not the problem.

 

I cleaned my 02 sensors today, and visually inspected my front exhaust tube. There was some rust on the covers, but the pipes inside looked good and could not have been damaged so having to replace the whole thing is stupid. I'll look up what this non fouler is. Thanks guys! Maybe try replacing the bank 1 sensor.

 

Visual inspection of your pre-cat on the outside tells you nothing about the catalyst on the inside and the actual functioning of the pre-cat.

Cleaning O2 sensors is hit and miss. I have seen some videos diagnose catalytic converters by measuring the temperature with an IR gun on the front of the cat and then on the back of the cat. Apparently, there should be little to no difference in temperature. If the cat is good then it operates/runs at a high temperature throughout. At least that is the theory....

 

The outside is meaningless as the catalytic action takes place inside the cat and with age, possibly overheated, melted, broken apart, etc the cat is done for. Have seen virtually new looking converters that were kaput.
One can always check the operating range of the O2 sensors through the OBD II port, but of course, if the downstream senor is out of range, one doesn't know for sure whether it's a bad sensor or a bad cat, but being it's on a Maxima, I'd say bad converter.

 

It also may be the MAF Sensor. if you have a OBD2 reader with live data see what the Flow rate is being reported by MAF sensor. I've had that before where due to bad MAF sensor Bank1 was reporting too lean but not Bank 2. O2 Sensor readings were fine. I will have to double check on a flow rate numbers but it should be somewhere between 9to12 at idle... don't hold me to those numbers thoe will get back to you with the correct numbers.

 

Well, hopefully it's not a bad front exhaust tube assembly. I'll keep you guys updated as things move along.

 

I would think that a bad MAF causing an incorrect mixture would trigger a precat O2 sensor fault, not post. Been quite a while, so hope I'm close, but the upstream sensor in closed loop one would see .1 to .9 volts aiming at a 14.75 A/F ratio.

 

defouler or sim

That's the only thing you need to know about fixing this. Replacing parts is stupid on a 3.0L.

 

I had the exact same problem a few months ago (May 2016). I initially reset the diagnostic code to check if comes back and it did in a couple of days. I reset it again and this time it came back after a week and I reset it again and it never came back. It's been 3 months now that the SES light is off and the P0420 diagnostic code is not getting generated.

When I look back, I can correlate now that the P0420 came as soon as I filled my gas tank with fuel from a pump that was substantially cheap. I did not think a lot of it then but I guess the SES/P0420 kept on coming back till all the fuel from that pump was burnt. Luckily I never filled gas from that that pump again and my Maxima is also not throwing any diagnostic codes.

I came so close to replacing the MAF/O2 sensor and also ordering a new CAT ...

 

As suggested above, defouler or simulator is the cheapest solution. However, replacing your pre-cat (this is what p0420 and/or p0430 is all about) has some merits because it's all but certain that your pre-cats are clogged. After 16 years, that's a given; the only question is "How badly?". I know because I was recently forced to replace my pre-cats. The result is amazing - the car gained so much power that I could not believe it.

I would consider this alternative only if your car is in excellent shape otherwise, and if you can't get away with defoulers or with gutting your pre-cats. If this is your situation, then look at CAT converters and tube assemblies at Rockauto.com; they sell them at 60% discount to the typical dealer price (for example, this B1 assembly http://www.rockauto.com/en/moreinfo....374533&jsn=368 is $262, your dealer wants $671). And yes, this one is OK for sale in California.

 

If you're going to take them off, then you can just clean them. They'll probably work, again, too.

 

or pull the plugs after you shut the car off and look for a wet one. If one is wet it could lead to a leaky injector. Could a bad regulator issue trigger the P0420?