2015 QX80 catalytic converter

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I just purchased from a law enforcement guy whose FIL owns the dealership. It has 155k miles but the selling point was the binder he gave me filled with every service record. Plus he gave me a good deal.

After 1500 perfect miles I recently took it to dealership to have oil changed, tires rotated and have a 100-point analysis done on it. ALL WAS WELL.

I drove it off the lot. The next morning the MIL light comes on solid and took it back to the dealership. I'm now told the "codes indicate BOTH catalytic converters need to be replaced..."

After asking a few questions I'm told it's ok to drive 1000 miles on it (I had another upcoming trip). I call up a different dealership (that the previous owner had do all his service on it) and gave him the vin#. I ask him if maybe the sensor itself or whatever is signalling codes to the computer might be checked first before shoveling $9k into the car. He talked around it a bit--focused his response on the full cost of diagnostics vs that cost being waived if I have the work done that the diagnostics suggest.

I drive mostly highway miles and although I don't know this car well yet it seems like it still drives the same but mileage gauge now sits consistently 16.9 vs 20+ that I was getting. (really...16,9? not 16.8 or 17.2) I drive with the mileage economy status window on that shows it fluctuating between 0 and 30 and it's at or very nearly 30 the entire time except when shifting gears (I'm in very flat country). While I'm not as concerned about mileage as I am about the care and maintenance, it is disconcerting.

Before buying I looked into QX80 issues and know that "things can happen" at this age and miles, etc. However, with all the catalytic converter drama the past few years, and this problem popping exactly right after it was serviced at a place new to me, I'm frustrated and a bit dubious.

Please share your thoughts and suggestions. Thx

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I should be more specific ---
Are there signs of actual cat failure that I should be looking for? I'm not a fan of "because the computer says so" but I'm also not a mechanic and only just learned what catalytic converters do...
Are there options or workarounds (as discussed on this forum and others that have worked for others with these codes) that I should try first before replacing the cats? Unfortunately, I am not a mechanic and will hire others to do any work but I'm hoping for guidance. Thx

[VStar650CL]

It would be helpful if you could post the actual codes. P0420/P0430 by themselves are usually good indicators of a dying cat, but when they occur along with other codes it can be a different story. Very often when you treat a cat code early, cleaners like Cataclean can prolong the life of the unit. You just don't want to push your luck. When the code starts popping up once a month or once a week it's time to replace it. There's a risk with any failing catalyst of what we call "cat inhalation", where the Miller cycling of the valve train causes the core of a melting cat to be sucked into the exhaust valves. That's uniformly fatal to the engine.

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Yes - thought about that after I left the computer. Thanks for reminding me:

I don't know if other codes were tripped--just going by the receipt I was given for the inspection.

The typical dlrshp service receipt indicates (comparing these to other receipts provided by the previous owner):

"Upon inspection of veh tech found 2 codes present for P0420 and P0430. Based on codes present technician recommended replacement on catalytic converter"

However, attached to the dealership receipt is another printout with no company header/info--just blank, identifies 3 names for "service Advisor" (w/ dlrshp email), "Parts Personnel" (w/ dlrshp email) and "Service Technician" (who is also ID'd on the dlrshp page but is listed with only a personal gmail address...)

and the quote reads, "Exhaust Catalytic Converter Replacement: P0420 and P0430 are tripped showing internal failure in both catalyst systems."
Itemized list includes two manifolds at $1500ea; 2 exhaust gaskets, one man exhaust gasket and two front exhaust tubes at $1500 ea.

A second page behind this one DOES have a dealership info at the bottom and has a quote for a broken door latch.

Thank you for that explanation. I have read other threads and you have mentioned those points, which I now understand better in context. I purchased some cataclean and will add it pretty soon. I feel frustrated by the sequence of events, since the codes tripped literally less than 24 hrs after driving the car off the lot.

My (limited) understanding is that the cats increase performance, fuel efficiency and exhausted air quality although my car does not require inspection in this part of the country. I'm curious how to determine decreased performance? Are the cats just losing performance and efficiency or are they actually clogged? Shouldn't the sensors and/or the O2 sensors be evaluated? Shouldn't the temp of the inlet + outlets be measured or back pressure test be evaluated first before replacing them? Or would these steps not align w/ garage protocol of the dealership?
Thanks

[VStar650CL]

If they tripped less than a day after you took possession, then chances are you got hosed and whoever sold you the vehicle knew it had bad cats. It takes time for P0420/P0430 to set. The amount of time depends greatly on driving circumstances, but they never return immediately. Sounds like someone cleared them long enough to sell you the car.

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The codes triggered about 1000 miles after I purchased the car. I suppose it's possible but that would be quite daring. The prev owner's name and address are on all service records (redacted but still readable). I have his number (called him several days after purchase). We met at his bank when I initially looked at the car and then at his church to complete the deal, which is all in a sparsely populated area so he would be quite easy to find again. He's also in state law enforcement. Still, the problems could have arisen and he decided to sell the car.

Since my previous post I added some cataclean, which immediately cleared the codes/MIL light. Days later, I was driving through a cemetery and accidently drove over a small branch and thought it might have knocked the undercarriage and either immediately or later that day the MIL light came on again. I've added two more bottles of cataclean and the light is still on - never went off again. I'm now on the road and I am worried it might not be a good idea to drive another 500 to get back home to get the issue handled.

Any thoughts on my final paragraph above and other ways to determine the actual condition of the cats?

Thanks for your responses.

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Sorry - need to clarify that the codes tripped after driving away from the dealership that had changed oil and rotated tires about 1000 miles after purchase (which was about 2 wks prior.)

I've read it doesn't take much to "poison" the cats simply by pouring something into the tank. Ironically, my gas door has a nice dent in it, which it didn't have when I bought it. There is also something that appears on the dealership's quote for cat replacement that I won't bother to repeat on this forum but that, plus the service man with the non-company email recommending replacement, the timing of it all, all these fantastic coincidences makes me lean more towards suspicion of sabotage.

If the cats are bad, I need to understand if they absolutely must be replaced in order for driving not to damage the car or cause more issues to arise that will require $ repair. I can manage expensive fuel fill-ups for a time period but do not want to do anything to damage the car. Plus, I need to determine if I can drive 500 miles or so to get back home before having anything done.

Before this happened I was hopeful to sell the car (it's GREAT just not what I think I want for road trips). But won't be selling it with an engine light on and/or bad cats.

[VStar650CL]

Cats don't melt very often, but when they do, because practically all Nissan engines are Miller-cycle, there's a good chance of inhaling the cat if a meltdown does happen. As I mentioned earlier, that's 100% fatal to the engine should it occur. VK56's aren't the champ for inhalation (that award goes to the QR25), but they are more vulnerable than the VQ series. So it's a value judgment, a very small chance of it happening versus a guaranteed catastrophe if you happen to roll snake eyes. Here's the result when a cat is inhaled, below is an actual cylinder on an inhaled QR25. The pink stuff is transmission fluid which the guy poured in thinking the cylinder was seized, not hopelessly welded:

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Great.

What is the likelihood that BOTH went bad at the same time?

I keep typing the beginning of questions that are really all the same question:
Where could I take the car to trouble-shoot possible issues other than melting cats?

[VStar650CL]

What is the likelihood that BOTH went bad at the same time?

Vanishingly small.

I keep typing the beginning of questions that are really all the same question:
Where could I take the car to trouble-shoot possible issues other than melting cats?

I assume you mean other cat issues besides melting, and there really aren't any which wouldn't cause symptoms. If a cat disintegrated you'd hear marbles in the exhaust pipe, if one got plugged you'd lose noticeable power. The only actual danger is from melting.

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I think I'm hoping to come up with other ways to investigate and confirm that the cats need to be replaced. One suggestion was measuring temps at intake + outtake, which should be +125F than intake. Another suggestion I read was to look into o2 signaling to make sure there isn't a wrong amount of o2 infused w/ fuel, which would trigger the same cat codes and could be just as damaging to the engine.

I'd like to make a few specific requests at a shop with the hope that I can trouble shoot other issues, and THEN if necessary, replace one or both cats. Thanks.

[VStar650CL]

The inlet/outlet temperature test is very reliable, but a little awkward to do on an Armada/QX80 without lifting it. Late model Nissan ECM's all cross-check the function of the wideband A/F sensors in front with the rear HO2S's, so it's very unusual for sensors to cause a false P0420/P0430 without generating other codes. The only thing which sometimes causes that is a pinhole exhaust leak in the immediate vicinity of a front or rear sensor. Other than that, when the cat codes occur by themselves. it pretty reliably means the cat is dying.

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OK thanks.

I think I'm going to have to find someone to lift it because it's still unclear which cat is bad. I'm going to bet that both aren't bad, despite the dealership quote for replacement of both (or are you supposed to replace both if one goes bad?) But will also ask for the temp test. How would one find a "pin hole"?

Your responses generate several other questions (the sign of a good teacher, btw!)

I came across a fantastic diagram someone drafted and posted and can't find it now but I was studying it to understand the sequence of function of these cats. I'll throw a couple questions at you -

I'm confused as to why an exhaust function (the cat is managing exhaust, correct?) would be subject to inhaling by the engine. Seems backward and that the exhaust would be pulling air from engine, not the other way around.

Also, aren't these cats designed and built from precious metals? Hence the cat thievery drama of a couple yrs ago. What in the world would cause parts to literally melt?

Also, I would expect in a true meltdown situation that other codes and alarms would sound---although maybe too late.

[VStar650CL]

P0420 is for the bank 1 cat (driver side on a VK), P0430 is for bank 2. If you have both codes then the ECM says both cats are bad.

Nissan engines practically all use Miller cycling to avoid the need for an EGR valve. EGR's work by injecting a small amount of exhaust back into the cylinders. Because that hot exhaust is devoid of oxygen, it paradoxically has the effect of reducing combustion temperatures and thus reducing the formation of nitrous oxides. In Miller cycling, the ECM manipulates overlap between the intake and exhaust valves to produce a minute "backflow" through the exhaust valves, thus retaining a bit of exhaust. It serves the same purpose as an EGR but without all the horrible plumbing. Great idea, right? BUT, the one huge disadvantage is that an EGR won't cause the engine to inhale bits of a melted cat. Miller cycling does.

Forget any warning if a cat melts. Other than a lit MIL (which you already have), the first warning you'll have is when the engine stops in its tracks because it's welded to itself.