Should i worry about a P0420 catalyst efficiency code with no symptoms?

[ElizabethFC]

My 2017 Civic’s check engine light came on last week, and the code reader says it’s a P0420 for catalyst efficiency below threshold. The weird part is the car runs perfectly fine—no rough idle, no loss of power, and fuel economy is normal. I’m just not sure if I should be worried about this immediately or if it’s something that can be monitored for a while.

[Aria.A]

I had a P0420 show up on a Civic once. The car ran fine—no rough idle, no power loss. The light stuck around for weeks and I just kept driving while watching for any other codes.

[JustinFH]

If there are no symptoms, it isn’t an emergency but it isn’t something to ignore either. It can be a bad downstream oxygen sensor or a real catalyst issue. I would plan a re-scan after a few days and check emissions readiness, rather than assuming it will fix itself.

[Anthony.G]

I swapped an upstream O2 sensor once thinking it would cure a P0420. It didn’t fix the code and I ended up chasing other random things; sometimes these codes don't map cleanly to one part.

[DanielT]

Have you noticed any other changes lately or any other codes pop up, even briefly?