P0420 on 2017 Honda Civic: DIY diagnostic vs mechanic, and gas-brand concerns.

[JohnR]

My 2017 Honda Civic's check engine light came on yesterday, and the car seems to be running fine with no noticeable loss in power or strange noises. I used a basic OBD2 scanner, and it's showing a P0420 code for catalyst system efficiency below threshold. I'm trying to decide if this is something I can troubleshoot myself or if I need to take it straight to a mechanic. For others who have dealt with this code, what were the most common culprits you found, and what diagnostic steps should I take before potentially replacing the catalytic converter? I'm also wondering if a recent fill-up with a different brand of gas could be a factor.