Is a P0420 Catalyst Code on a 2018 Civic Urgent or O2 Sensor?

[JerryLC]

My 2018 Honda Civic's check engine light came on yesterday, and the car seems to be running fine—no rough idle, no loss of power, and fuel economy is normal. I borrowed a basic OBD2 scanner from a friend, and it's showing a generic P0420 code for "Catalyst System Efficiency Below Threshold." I'm trying to decide if this is something urgent I need to take to a mechanic immediately or if it could be a less serious issue like a faulty oxygen sensor. Has anyone dealt with this specific code on a similar vehicle, and what was the actual cause and repair cost in your experience?