Code P0420, hoping for a cheaper solution?

2016 BUICK VERANO
98,000 MILES • 2.4L • 4 CYL • 2WD • AUTOMATIC
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XXCLOUTCOBAINXX
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I'm finding myself in a pickle and would appreciate advice. I drive the vehicle listed above with less than 100,000 miles. And recently I got the dreaded p0420 code. Granted this isn't the first time. The first time was appx 2 years ago, with 20kish less miles.
Spark plugs were shot, replaced them, threw in some cataclean, and used a bottle of CRC GDI Intake Cleaner, and we were good. Light went off for two years. I've changed spark plugs one more time since then, especially because I put flex fuel into her a few times thinking she was a flex fuel vehicle (oops)

But it's been years since that has happened.
Anyhow, this time around I've tried everything. New air filter, crc, cataclean x2, flooring it, long highway drives, premium fuel, everything short of replacing actual parts. The spark plugs are fine, but they are far more black than they should be with how old they are. Any suggestions? I'll leave a photo of some live feed data. Please tell me it's not the cat (I live in California and there are no approved aftermarkets I can find, aka need a $3000 cat). Please help.
Apr 5, 2025 at 10:33 AM
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XXCLOUTCOBAINXX
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Here is one pic of live feed data, idk why it didn't attach earlier!
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Apr 5, 2025 at 10:34 AM
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XXCLOUTCOBAINXX
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Also forgot to mention my car is originally CARB compliant so if it is the cat, I would need to replace through Buick and dear god I might as well buy a new car? And also, the fixd data for the sensors is 1/2 being upstream/downstream. Idk why they did it like that.
Apr 5, 2025 at 10:54 AM
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STRAILER
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The car seems like it is running rich, this is why the Short trim is -21 which means the cat can be broken down inside and lowering the engine vacuum, I would remove the cat for inspection and if it has fallen apart inside there is no fixing it. Here is a guide that goes over a rich fuel condition as well to help understand what's going on:

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

Here is another guide to help confirm the issue:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Here is one for $126.00 from Amazon:

https://amzn.to/4j0kvsN

Can I confirm the engine is running okay? Check out the images (below). Please let us know what happens.


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Apr 5, 2025 at 2:46 PM
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STRAILER
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Here are the companies that make cheaper cats for it:

Brands: Walker, Eastern Catalytic, MagnaFlow and Bosal.

Here is one for $126.00 from Amazon:

https://amzn.to/4j0kvsN

Please upload pictures or videos in your response of any problems.

Apr 5, 2025 at 2:53 PM
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XXCLOUTCOBAINXX
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Hi! So, the car actually runs a little rough. Always has. It gets better when I change the spark plugs, which is WAY too often for iridium (just learned that today), that pic in the first link with the black spark plugs is what they look like after 20-40K miles.... I wonder, could a bad PCV valve cause an engine to run rich? These vehicles are notorious for those going bad. And now I also have either leaking or burning oil, burning by the smell of it. So many issues I'm finding that all seemed to be caused by one thing

And unfortunately, I'm in CA and no aftermarket manufacturers *approved by CA with a License # linked to my vehicle type* make CARB complaint cats for my vehicle. Which leaves me with only *non used* OEM options. Aka $2,500 for the part.
Apr 5, 2025 at 3:59 PM
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STRAILER
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Hmmm bummer, yes, the PCV system can make the engine run rich, here is the location of the hose, the valve cover has an internal system which handles the gases. With the engine running, remove the oil fill cap to see if there is suction or pressure present, please upload a short video of what you find.

Apr 6, 2025 at 1:04 PM
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XXCLOUTCOBAINXX
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So there is puttering pressure. Same with the oil dipstick. No sludging on the oil cap. I do have a seal leak, had that checked for just in case the rear main was blown. She's all good, but you do have a front block leak.
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Apr 6, 2025 at 4:39 PM
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That looks like the crankcase ventilation system might not be working from what I could see. Thanks for the video. It seems like the catalytic converter needs to be checked, if it is partially clogged it will make the engine run rich.
Apr 7, 2025 at 10:50 AM
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XXCLOUTCOBAINXX
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Could it be piston rings? I'm hoping not. If I have to pull the engine I'm screwed. But have done a deep dive and it seems that's a common issue with this car is piston rings fail causing blow back. On top of the PCV system clogging causing further crankcase pressure. And both of those ruin spark plugs and the cat which causes even more pressure up until the seals blow (my front block seal is shot, held together with a bottle of Lucas currently). I put a vented oil cap on yesterday and the car is running so much better. But please, from a frugal standpoint, and with relatively decent mechanical knowledge and unwavering determination. What can I do myself? Replace cat? Piston rings? Valve cover and gasket? Remove intake manifold to unclog PCV system? All four of those I think are musts, but I don't have any schematics or specifics on this car to do it.
Apr 8, 2025 at 12:27 PM
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STRAILER
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I would definitely remove the valve cover and clean/replace it. Also, let's do a compression test.

Here is a guide to help:

https://www.2carpros.com/articles/how-to-test-engine-compression

Please start a new thread for any instructions needed. Please go over this guide and get back to us.
Apr 9, 2025 at 10:18 AM
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XXCLOUTCOBAINXX
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Compression test went as follows:
Cylinder 1: 135 PSI
Cylinder 2: 140 PSI
Cylinder 3: 135 PSI
Cylinder 4: 130 PSI
All on the lower side, nothing crazy though, right?
Apr 9, 2025 at 6:28 PM
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STRAILER
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Yep, these seem okay. We have gotten away from the catalytic converter issue, let's start a new thread to continue.
Apr 11, 2025 at 9:51 AM
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XXCLOUTCOBAINXX
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Sounds good. Uh, what should I say to start the new thread?
Apr 11, 2025 at 12:03 PM
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STRAILER
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"black spark plugs after 20-40K miles" Is what I would put as the title.
Apr 13, 2025 at 9:22 AM