Need help w/ 0P420 code
I have a 2010 Hyundai Elantra with 341,000 miles on it. After driving with a misfire for too long, last year my cat got poisoned and I replaced it with this catalytic converter from Magnaflow. The cat was replaced in August of 2025 and up until 2 months ago I had no codes. I had 2 codes happen at that time, P0441 and p0444. I replaced the solenoid valve and the code went away and the car had no codes and ran fine. A week ago, I went to go get smogged and used my code scanner to make sure it was ready and I had 2 pending codes show up. Codes p0456 and p0420. I was already familiar with P0420 but 456 was new. I checked my gas cap and couldn't figure out what else to do so I disconnected the battery overnight, reconnected it in the morning, drove it for probably 50 miles and the check engine light came on again, and it's the 420 code.
Other than a bad cat, what could be other causes? What else should I look at to figure this out? Oxygen sensors?