What is the best diagnostic sequence for P0420 on a 2015 Civic?

[GeorgeJL]

I drive a 2015 Honda Civic, and my check engine light came on yesterday with no noticeable change in performance. I borrowed a basic OBD2 scanner, and it's showing code P0420, which points to a catalyst system efficiency below threshold. I'm trying to diagnose this myself before taking it to a mechanic. For others who have dealt with this code, what's the most logical troubleshooting sequence? Should I start by checking for exhaust leaks before the catalytic converter, or could it be something simpler like a failing oxygen sensor? Are there any specific tests with a multimeter or visual inspections I can do to narrow it down, and is it ever safe to drive with this code for a short period if the car seems fine?