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P0420 code still coming back

I own a 2010 GMC Yukon XL Denali that keeps throwing a P0420 code. I am at the point of just getting rid of it because we can’t seem to get any help actually getting it fixed. It started throwing this code a few months ago. We were told that we should try a new catalytic converter so we had one installed. Still the code came back within a day. We have since replaced both bank 1 O2 sensors also because they were cheap enough and I wanted to rule out that they could be the issue. Nope! Within a day or so that pesty P0420 code is back just as bright as ever. We have had it in 2 local Meineke repair shops now and they say I’m crazy and there are no exhaust leaks. The 2nd shop said my vehicle is running rich and to use seafoam and fill up with E85 and it should help. Nope! It has never shown any other symptoms other than the P0420 code.

But I still here some kind of almost bubbly kind of sound when accelerating and going up hills coming from my front left towards my emergency brake, but obviously outside the vehicle. I’m at a loss. What’s some opinions on what I should do from here?

Which catalytic converter did you replace? You have three of them. Each bank has a warm up cat, they merge into the final cat. The O2 sensors may be measuring the warm up cat on bank one only, but usually the bank one cat and the final cat are sold as a single unit.

I replaced the top one

in the photo. It was replaced with a magnaflow 99106HM.

Exactly how did they verify there are no exhaust leaks?

Was it Meineke who initially diagnosed the P0420 fault code?

If they claim you’re crazy, how do they account for the code coming back?

In any case, it sounds like that may not be the place to go for a proper diagnosis and repair.

They didn’t tell us how they tested it. They just said there was no leaks after they had charged us over $500 to keep “adjusting” the exhaust. They said we would have to pay an additional $100 to do a diagnosis of the code. So we took it to a different Meineke that seems more knowledgeable and they said they sealed 2 pinhole leaks and that my car was running rich but to do the seafoam and E85 has and the code should clear.

I suggest taking the truck to a reputable independent shop

Tell them everything

You had P0420 code

You replaced both bank 1 oxygen sensors and the y-pipe assembly

Yet the codes keep coming back

You’d like an estimate to repair

Another thing . . . if they do find the problem and repair it, politely insist that they clear the code and then drive the truck as long as needed, for the catalytic converter monitor to run to completion. That is something the other shops probably did not do, that is why they handed you the keys without verifying the problem is indeed resolved

Don’t name the other shops, unless they specifically ask who worked on it. Badmouthing another shop isn’t always the best idea, for various reasons


Looking at the picture you provided, I see bungs just behind the warm up cats (the front ones). Are they for the front O2 sensors? If so, they are not being monitored for P0420, only the rear. If they are for the secondary O2 sensors with the primary O2 sensors in the exhaust manifolds, then that was the correct cat to replace.

Edit: on second look, it appears that there are 4 bungs so the primary O2 sensors should be just in front of the warm up cat and the secondaries just behind.

You can check for an exhaust leak yourself. Have someone hold their hand to the rear exhaust pipe trying to seal it, with the engine idling. You will hear any leaks up at the engine area if there are any. Candidates for leaks would be a cracked exhaust manifold, exhaust manifold gasket, leaky dough-nut gasket. You will hear a pfttt pfttt pfttt… sound at the leak.

I am having a little trouble accepting that they would 'seal ’ a leak and replace the part that had a pinhole .

I assume this is a ’ Flex Fuel ’ vehicle but would you not recommend using fuel with only 10% Ethanol with the Seafoam .

I would never tell anyone to use Meineke for anything.

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I hope your vehicle is Flex Fuel, otherwise that suggestion to load up on E85 is going to do a ton more harm than good…I agree with @db4690, you need to take this to a good independent mechanic, give them the entire story, and let them get to work. Unfortunately, chain shops are rarely going to be the best place to take a vehicle for diagnosis or repair work (especially chain transmission shops…)

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Sorry I forgot to include in the original post that they also replaced exhaust manifold and gasket. I will have my husband plug up the tailpipe and see if we hear anything. Thanks for the suggestions! Someone suggested replacing the map sensor to help with the rich fuel. We did that last night and so far no light, but not sure if it really will help.

Note, he doesn’t have to completely stop the exhaust, just create some additional back pressure.

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The replace this, replace that technique brand of auto repair worked before the switch to electronic fuel injection systems, but with modern engine designs like yours you can easily run out of money before you run out of ideas of what to replace next. Suggest to focus on getting a correct diagnosis instead. Chain auto repair shops are good for some things, oil changes, muffler replacement, etc, but not for problems related to the computer controlled emissions system like a p0420. Start by asking your friends, relatives, co-workers which independent shop in town they use for repairing their cars. You don’t need an exhaust system specialist, just a good repair shop. Once you’ve identified one, tell them all the background, then just let them solve the problem for you. If your budget only allows so much, tell them in advance.

As far as what’s causing the p0420

  • Maybe it already is solved, and the CEL just needs to be reset
  • The shop fixed part of the problem by replacing one cat, but the other at needs to be replaced too
  • Something is causing too much fuel to be injected which is damaging the cats or confusing the O2 sensors. That should throw a rich code, or at least show up in the fuel trim data. If the codes and the fuel trim data confirm overly rich operation, check for pre-cat exhaust leaks or a vacuum leak. You mentioned you heard a “bubbly” sound going uphills and accelerating. That could be an exhaust leak, but it could be a vacuum leak too. Or it may the engine pinging a little.

In summary, the least expensive solution is most likely to give up on the replace this, replace that method, and focus on finding a shop who has the gmc scan tool, training, and experience to solve it for you. If no such inde shop exists in your area, you’ll have to go to a dealership shop.

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It is a flex fuel. They supposedly sealed 2 pinhole leaks on top and bottom on the weld where the new cat was installed.

All 2010 Yukons are flex fuel vehicles.

I don’t like those universal fit, weld in catalytic converters. I would have installed a direct fit assembly like the one in your picture above.

Now you are dealing with leaky welds and $500 in adjustments? Doesn’t the installer warranty their work?

Time to take the vehicle to a professional repair shop for a proper diagnosis.

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I just reread the postings

The picture of the y-pipe threw me for a loop. Now I see that the y-pipe wasn’t replaced, the shop just welded in a new bank 1 cat, and apparently didn’t do a very good job

It’s not clear to me if Meineke welded in that cat, or merely performed the “diagnosis” and sent the customer on their way. If they welded it in, that’s really scary. That would imply they’re not even good at exhaust repairs, which is their specialty

In any case, we’ve replaced a few such y-pipes for our fleet vehicles. Often due to theft recovery. In any case, the price isn’t cheap, but it’s worth it to spend a little more for the y-pipe, versus just welding in one cat. And it’s easy to do.

Of course, the pinholes also make me wonder if Meineke did an especially poor job, or maybe the exhaust system is so corroded, that the entire y-pipe should have been replaced, versus just the bank 1 cat

Believe me I’m regretting the decision to save money and try the cheaper weld in cat. It’s ended up basically costing the same and the issue isn’t even fixed. They don’t want to cover anything because they say there is no issue with the work they did. Sounds like it’s time to take it to a shop.

some time ago I was dealing with P0420 followed by other codes as I proceeded with catalyst replacement in my 2006 Pathfinder.

I strongly suspected a leak in exhaust system and asked a “local independent shop” to perform a smoke test to find it

long story short: they lied repeatedly they performed a test and “it was fine”, and went on “lest replace everything on the car, milking owner in the process” journey, but I was able to find multiple leaks and bring them a video recorded with smoke leaking all around… at which point the owner refunded me 100% of what they bled from me by that point and angrily placed me on some “shared computer database” as a “bad customer” :slight_smile:

anyway, if you can not get satisfaction from the shop what made the repair - get another opinion and you might even go back and demand refund if another shop finds glaring incompetence like I did

if your hands are itchy, you can look at YouTube for “DIY smoke machine” for ideas, I’ve made mine from the old paint can, wick and some resistance wire, plus some fittings and hose


Thank you much! I was beginning to think I was crazy.