Check engine light came on this week and I had it scanned at Auto Zone and only one code popped up "error P0420" and shows probable cause as:
1) large vaccum leak
2) rich AF ratio - fuel system faulty
3) Ignition system malfunction
4) Faulty catalytic converter

Anybody had this issue or have any possible solutions. Auto Zone tech said probably bad gas but I doubt that's the case.

Thanks in advance for any advice...

I would research the code further in the factory service manual starting on page EC-267. www.nicoclub.com/FSM/

Your code is for bank 1 so that's the left side. I would go through the dignostics and see where it takes you.

Heath

Thanks Heath, you've been a good reference.

Are you opposed to having a Nissan/Infiniti dealership fix this problem? Or are you just trying to save some money and DIY?

Well I can tell you that based on a lot of other posters, I want to get a flavor for some of the problems these vehicles have before I take it to a dealer who charges $100 just to diagnose and then who know how much. I may take it to a dealer once I get a good grasp on what needs to be done and read some of the TSB's. I runs well just has a lot of miles and I want it to last another 50k miles.

Do you by chance have a small exhaust leak? It would be on the drivers side most likely.

I do not think that what you have is going to be a DIY one.

The PO420 is a base catalyst efficiency dtc....basically long story short your engine computer is seeing a bunch more O2 in your drivers side exhaust stream than it should see, it is trying to counteract that by adding more fuel as it would normally but it is not enough.

For your sake I hope you have <80k on the truck, as that should still be covered under the Federal 8/80 warranty. If not you will almost certainly need to have the exhaust manifold and catalyst assembly replaced soon.

Steve, thanks for you post. I just read and replied to your other post and appreciate the help. I am getting the rotors replaced this week to hopefully help with the brake issue and will have the exhaust checked by a dealer in the coming weeks.

Hope to have this truck running like a top before long. Take care and thanks again.

SteveTheTech wrote:Do you by chance have a small exhaust leak? It would be on the drivers side most likely.

I do not think that what you have is going to be a DIY one.

The PO420 is a base catalyst efficiency dtc....basically long story short your engine computer is seeing a bunch more O2 in your drivers side exhaust stream than it should see, it is trying to counteract that by adding more fuel as it would normally but it is not enough.

For your sake I hope you have <80k on the truck, as that should still be covered under the Federal 8/80 warranty. If not you will almost certainly need to have the exhaust manifold and catalyst assembly replaced soon.

Steve,

As I was going home last night my SES light was not on and I have about 1/4 tank of gas. The SES light did not come on until after I filled up the tank when I first purchased it and I put 86 octane in. Could this have happened because I did not fill up with premium? Just curious because the light is now off and I haven't done anything out of ordinary to fix????

I find it unlikely that low octane ratings will cause a P0420. I personally think the half of the engine is running as rich as it can. The reason it is rich is the crack in the exhaust manifold. The positive pressure pulses are expelling unprocessed fumes while there is a vacuum pulse on the other end of the pulse this intakes enough O2 to skew your Air Fuel Ratio sensor.

Are cracked exhaust manifolds a common problem with the QX56 engine? If so have the OEM replacements been upgraded so it doesn't reoccur?

It's not as common as older domestics in New England used to be but I had a sneaking suspicion right off the bat. They are pretty easy to diagnose, not easy or cheap to fix though. But I have not seen a repeat.

SteveTheTech wrote:I find it unlikely that low octane ratings will cause a P0420. I personally think the half of the engine is running as rich as it can. The reason it is rich is the crack in the exhaust manifold. The positive pressure pulses are expelling unprocessed fumes while there is a vacuum pulse on the other end of the pulse this intakes enough O2 to skew your Air Fuel Ratio sensor.

I am now wanting to get to a dealer for inspection as soon as practical. Is a possible crack in exhaust manifold an urgent issue? Could it be an exhaust leak under the driver side near cat as well? Thanks Steve for your insight...

Diagnosed truck today at Nissan Dealership and they quoted said I need to replace my drivers side catalyst which is why the check engine light is on. Quoted me $1125. Can I not replace this at an exhaust shop for a fraction of that?

The catalyst is integrated into the exhaust manifold, so it's not available as a separte part from the factory. I'm not sure if an exhaust shop could weld something on that would work. The manifold and gasket is available for about $490 from Infiniti of Scottsdale. You can check out how involved the repair reading the FSM at www.nicoclub.com/FSM/ starting at EM-18.

Heath

Once again, thanks Heath. Excellent resource!!