Nissan XTerra catalytic convertor/O2 sensor issues....

At a loss on this this, need help.  Nissan dealer tech said sometimes the ECU needs to be reset after cat replacement, which was a new one for me.  Guy was super cool and did it for me.  Warned me if this didn't work, that means the ECU isn't happy with the new cats and they might need to be replaced again.  Great.  Well, the light came on a week or so later.  Is this something people have dealt with before?  I've never had to replace cats on any of my German cars and an O2 sensor code on one of those means the O2 sensor is bad, not that the cat is bad.  So WTF?  This should've been a simple parts replacement and now I've got a persistent CEL.

I'm in the unenviable situation of having bought the cats from one shop and had them installed at another.  I've reached out to the first shop, who's owner I've known for years btw, and he's not really helping me out.  I want him to warranty the passenger side cat for me.  I guess I'll be stopping by his shop tomorrow to talk to him in person about this.  It's BS that I bought the cats from him and he's not willing to do any warranty support for me. 

Another idea I have is to use an O2 spacer on that passenger side primary O2 just to see what happens.  I don't want to run it long term but if it makes the ECU happy then that tells me that cat does need to be warranty replaced.

Is it the upstream or downstream O2 sensor that is throwing the code?  The upstream O2 sensor should be agnostic to the catalyst, and you definitely don't want to mess with it, since it is responsible for your fuel trim.  If upstream, it was may have died during the cat replacement.  They are really fragile and suceptible to vibration damage when hacking out the old cats.  Another possibility is that there is a gasket issue and air is getting sucked into the flange at the new cats, which is fooling with the readings.  

If downstream, still susceptible to damage and air leaks.  Downstream O2 is just to verify catalyst efficiency.  An aftermarket catalyst may not be efficient enough to satisfy the downstream sensor, and a spacer would be fine for the downstream sensor if that turns the light off.

What's the specific code you are getting now? If it says catalyst efficiency after a catalyst change, that would be odd- as that means the front and the rear sensors are tracking each other close enough to detect catalyst damage.  If it's a sensor error, that would be a different problem.  Systems are designed to detect multiple things- there are some intrusive tests to make sure both the front and rear sensor are working properly.  

So please post the specific code that is lighting the light- that will tell us more what is going on.

BTW, Tyler- the rear O2 sensor is used for fuel trimming to optimize the emissions.  That operation is pretty equally as important as it's function as a catalyst monitor.  The amount of emissions reduction it's capable of dealing with is pretty important- and for some WBO2 systems, it's also one of the most important sensors to control emissions.

In reply to Tyler H (Forum Supporter) :

So the cats were replaced with Walker direct fitment Cali compliant cats.  A few weeks after that was done it threw a code for the upstream passenger side O2 sensor.  I went to replace it and found that the shop had cross threaded the O2 sensor and I couldn't remove it.  I took it back to them and they were able to remove the O2 sensor and retap the threads, then they installed the new Denso O2 sensor I had. 

Now it's throwing the same upstream passenger side O2 code.  I took it back to the shop that installed the cats, they basically either don't know what's going on or don't want to deal with it (Midas, not the dealer).  Dealer reset the O2 values in the ECU but warned me if the light comes back on that means the cats may need to be replaced again.

I can go ahead and replace the O2 sensor again and return the "new" one back to the Auto Parts store but unless I just got unlucky and got a bad sensor out of the box it seems like the cat itself may be the issue?

I would strongly recommend getting the actual code and checking in a Nissan manual what it means. Most people read the very short precis of the code and don't go further, but there can be a lot more information in the actual code definition and a factory manual will give you the best data to work from. There can be a dozen "o2 sensor" codes, and they all mean different things.

Upstream could be a heater problem, which could be as simple as someone not properly reattaching a ground. Get the real code.

Decade long ASE master tech, former shop owner, currently work on Nissans for a living. Seconded on needing a code to provide good insight but aftermarket o2s are maybe a 50% success rate on Nissans. After market cats in general aren't super effective, the less expensive the cat the less of the material that does the job they contain. 

Cats should have nothing to do with upstream o2 codes but I've seen all sorts of craziness over the years. 

In reply to Keith Tanner :

Lots of this.  There are so many O2 related codes, it's hard to guess.  Not sure what year of vehicle this is, nor what emissions standard it was certified too- but there was a quick advance of front sensors from '02 to '10- with more and more powerful heaters and then a transition to front WB sensors (with more and more aggressive heaters).  Along the way, the fact that different manufacturers had slightly different transfer functions can really make it tricky to replace everything.

It's a 2011.  I'll pull the code and post it but as I recollect it's an O2 sensor code, not a cat code.  Yes, I know the aftermarket cats aren't as good as OEM but I've had good luck with them for at least a few years.  I'd be surprised if the cat was bad out of the box.  I suppose I could've gotten a bad O2 sensor out of the box but I bought a Denso one on purpose vs some other crappy aftermarket one.  There's just the one plug for the O2 sensor, no separate grounds for it...

Upstream O2 code will not likely have anything to do with the cats.

X3 on needing the code, and it's unlikely an upstream a/f sensor code is due to the cat.

The Nissan tech should have done fuel injection active test with his Consult. Basically you can force the fuel trims +25% and -25% and compare readings on all four O2/AF sensors upstream and downstream, bank to bank. It should be readily apparent if you have a skewed H02S2 causing an A/F sensor code or if the aftermarket A/F sensor isn't reading right.

In reply to docwyte :

2011 is borderline WB front sensor... although if it plugged in, then it's the right one, LOL.....

Denso makes good parts, but it's possible that you just got some bad ones.  The fact that it plugs in suggests it's the "right" one, as we all have keyed the connectors to not be able to put the wrong one in.  Odd. 

(and yea, of that era, the grounds are totally though the ECU- all 4 wires for an NBO2 go to the computer)

I am leery of any aftermarket converter.  Have had far too many where they had leaks where they were not welded fully or in the right spot and they needed to be corrected before installation.

The fact that whoever put it in mangled the oxygen sensor and didn't try to fix it suggests that it is doubtful that they were conscientious enough to make sure the converter assembly was correctly manufactured, gasket surfaces clean, etc.

Ok, just read the code its:

P2A00, O2 sensor circuit Range/Performance, Bank 1, Sensor 1

Google spits out either O2 sensor replacement, exhaust leaks, bad MAF (??).   I suppose it could be a bad new sensor, easy enough for me to pull it and have the auto parts store warranty replace it.  At the same time I can check the cat there for leaks...

You will need to monitor some data with the engine running, but from what I am reading it is not uncommon for a failed/skewed downstream sensor to cause this.  Or a bad new sensor.

My gut feeling is still that there may be an exhaust leak somewhere upstream of the downstream sensor.  But if they cleared all adaptives with a skewed downstream sensor, the computer is probably going to assume that the upstream is faulty since it is the more delicate one.

V engine, right?  So that rules out the MAF, as it would change both banks at the same time.

Given that the tech cross threaded the sensor, there's a really good chance there's a leak somewhere, like Pete suggests.  This is where the new cat will have an influence- if it was installed with the same care as the cross threaded sensor, it's possible that the gaskets are leaking, too.

In reply to Byrneon27 :

How good are you with the Z-24i engine?   Most Nissan techs won't touch them.  

In reply to alfadriver :

I cheated, I hit up Identifix once I got the code   Well over a hundred hits for upstream sensor, although a couple of them noted in the comments that they replaced the downstream.  A lot of comments about bad new in box.  57 hits for downstream.  A lot of hits for converters, mostly with rust holes in them, and a few hits for leaky downpipes.

I cannot remember if this is one of those that has the nightmare downpipe that likes to rust through a lot or develop a bad flex joint, and is part of the rear converter assembly.  Rear converter is not monitored but I cannot remember where the downstream O2s are located. 

BNIB always worries me.  It is one thing when you are a professional and start to doubt your own diagnostic abilities, it must be another entirely for a DIYer to throw a new part in it and the same fault comes back, and immediately think "No that wasn't it..."  I always hit a drop of Stabilant on air/fuel ratio sensor connectors anymore, the sensor readings are in the microamp range and it does not take much connectivity issue to cause problems.  Stabilant is kinda expensive for a DIY'er to buy, though.  A 1oz bottle was like $120, we bill it by the drop.

I've tended to go with OEM O2s, or at a minimum the OEM's supplier, which may actually be Denso in your case.

In reply to dj06482 (Forum Supporter) :

That's why I went with Denso, it's the OEM supplier.  OK, well I guess I'll go ahead and replace the upstream O2 sensor again and then check the flanges on that cat for leakage.  The gaskets were replaced when the cats were installed, I don't hear an exhaust leak.  If I still get the code then I guess my next stop is the dealer as I don't want to load up the parts cannon on this thing and keep going.

spitfirebill said:

In reply to Byrneon27 :

How good are you with the Z-24i engine?   Most Nissan techs won't touch them.  

 

I'm of exactly no help sorry to say, traded fun day to day work for a 401k and health insurance. (work at a dealership) don't think I've touched anything older than a 2010 in a few months. 

Bringing this back up.  Truck went to a Indy shop a maybe 10 days ago.  They smoke tested it, no leaks.  Replaced the O2 sensor with an OEM one from the Nissan dealer.  Said the ECU is reading the O2 sensor, resistance values are normal, they forced the ECU into test mode to test the O2 sensor and it was fine. 

So the check engine light is back on.  Dammit!!  P2A00 code this time, which is slightly different than the last time.  Called the shop, it's going back to them next Thursday, they were willing to have us drop it off before that but my daughter needs a car.

So WTF is going on here?  I've never had a car throw persistent O2 sensor codes like this.  They throw an O2 code, I replace it and it's gone for good.

Now this car has had two different O2 sensors, clearly that's not what is causing the code to set, the shop smoke tested it and there are no upstream leaks that would trigger this.  What else can it be?  It won't pass emissions with this code set, so I've gotta get it fixed....

What are the chances the wiring has been slightly damaged and causes intermittent errors? Is it easily accessible to be inspected?

Also I read somewhere that a bad MAF will throw this code as well. Not sure how accurate that is, but might be worth looking into.

Wow, It's gotten a lot worse than when I had it. I'm going to toss this one out there as it's something that happened to me with the truck. Double check the grounds.

When I first got to Colorado with it, The Ex and I were heading up Gold Mine there in the Springs and it got to the washboards in the dirt road and The truck just spazzed out. Stalled and took a minute to do anything then was fine. One of the ground wire connections came loose. I replaced one ground and tightened the rest. I wonder if it is coming back only now with the O2 sensors. 

Just a thought since everything else was done to it. 

Good video: https://www.youtube.com/watch?v=zI5qyzzh608

In reply to bmw88rider :

Thanks, I'll have them check all that.  You mentioned you regretted selling it, want it back?