2013 Hyundai Accent 255K miles. Runs great, still gets around 35 MPG. 40K miles ago I was having trouble with 420 code. Did usual tune up stuff and both O2 sensors. Ended up replacing the Cat with an aftermarket from Eastern Catalytic. I noted that the old one had no carbon or scale, I've seen very nasty Cats before.
When I changed the plugs about 15K ago they were normal, no heavy fouling. Now I'm getting the code again after only 40K miles. I think it's a false code caused by something else. What's odd is if I clear the code with a scan tool, turn off the key, turn it back on, and check the scan tool, it still has 2 420 codes, 420 generic pending. I think this is weird since I haven't run the car after clearing it. I did some work recently and had the battery disconnected for 4 hours and the code remained.