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2007 Hyundai Elantra GLS -- bought it used and previous owner said he installed a new catalytic converter before I bought it from him. Passed smog when I bought it. About 6 months after I bought it I started getting P0420 -- Catalyst System Low Efficiency. I bought a new downstream O2 sensor, thinking maybe the sensor was faulty, but when I put it in I started getting P0172 -- Fuel System Too Rich (Bank 1), so I swapped the old one back in. I know it probably wasn't the best thing to do, but with the old O2 sensor, I would get the following behavior:

  • No code until I drove ~100 miles, until I started commuting to a new office
  • I would reliably get P0420 after driving to my office the 2nd or 3rd time, which consists of driving 2 minutes from my house to the freeway, 10 minutes on the freeway at 75mph, 2 minutes from the freeway to the office. P0420 comes on just as I'm about to get off the freeway. I would reset the code when I got home, and it would come back the 2nd or 3rd time to the office.
  • Once, P0101 -- Mass Airflow (MAF) came on, and I installed a new MAF, and I didn't get P0420 for over 2000 miles. Now, I'm back to the same pattern

I looked at the PCV valve, and it still rattles. I'm about to swap out my upstream O2 sensor to see if anything changes. After that, I'm thinking to swap out my fuel injectors, since they look a bit old. I could swap out my MAP sensor too, since those are cheap.

I'm willing to pay the ~$1000 for a new catalytic converter, but something tells me that the problem is somewhere else -- any thoughts?

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  • Was there any clue given as to how he broke in the cat? If not done correctly, it could be the "new" cat ain't working as good as it should. Dec 1, 2023 at 0:41
  • oh wow I didn't realize there was a breaking-in method. Thanks for the link. I actually don't know how the cat was broken in, but now I'm starting to think maybe I indeed need to buy a new one :(
    – richyen
    Dec 1, 2023 at 7:05

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So I ended up swapping out the catalytic converter and the P0420 has gone away. I was surprised how easy it was to take the old cat out on this model car, and it turns out that a huge chunk of the ceramic honeycomb was broken off, which matches the Catalyst System Low Efficiency description of the P0420 code.

Now that I know how easy it is to take the cat off the tailpipe, I can just visually inspect it in the future, should the P0420 code ever come up with this new cat 👍🏼

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