Should i worry about a P0420 code on my Civic?

[Riley_M]

My 2018 Civic’s check engine light came on yesterday, and the code reader says it’s a P0420 for catalyst system efficiency below threshold. The car seems to drive completely normally, no odd smells or sounds, and my fuel mileage is the same. I’m just not sure if this is something I need to drop everything to fix immediately or if I have some time to figure it out.

[NoraEG]

That P0420 is one of those things you notice more than you feel. It means the catalyst system isn’t hitting the efficiency the ECU expects, but there can be a lot of causes from a bad downstream O2 sensor to a tired converter. I drove mine around for days with the light on and no obvious symptoms, so I didn’t panic, just planned to have it checked when I could swing it.

[Benjamin_M]

I had the same light on my Civic last year. I tried clearing it once and it came back. A shop told me the likely culprits are the downstream oxygen sensor or the cat itself. I ended up replacing the sensor, drove fine after, and the light stayed off for a while, but it did come back later once, and I didn’t chase it further.

[Joseph_G]

Feels odd to worry about it when it runs smoothly. I kept thinking maybe it was just a hiccup, but who knows. My fuel economy stayed steady, and there were no smells or rattle. It’s easy to brush off, but I’m not sure what’s really going on under the hood.

[Brian_G]

One question, if you don’t mind: do you have any freeze frame data or live sensor readings from your reader? If not, I’d probably just keep an eye on it and maybe schedule a check before a long trip, but I’m not sure this is the real problem.