What should I check first for a P0420 catalyst efficiency issue in my Civic?

[Savannah_L]

My 2016 Civic’s check engine light came on, and the code reader says it’s P0420 for catalyst efficiency below threshold. The car runs fine, no odd noises or loss of power, but it just failed its state emissions test because of it. I’m trying to figure out if I should just replace the whole catalytic converter assembly or if there’s a chance it could be something simpler like an oxygen sensor causing the reading.

[IsabellaEB]

P0420 shows up a lot when the post-cat sensor or the catalytic converter itself is acting up. In my Civic the code came back after a while and I found the downstream O2 sensor was flaky; replacing that sensor fixed the light for several months. If yours runs fine and you pass the idle test, it's not necessarily the whole converter that's bad.

[ZacharyB]

I wouldn't rush to a full converter replacement. These codes wander around because sensors wear out or things like leaks sneak in. My car felt normal even when the light was on, which made me doubt the worst-case scenario.

[MasonC]

One time I chased it for weeks and finally found a tiny exhaust leak near a gasket that was throwing off the downstream readings. It wasn’t the converter at all, and after fixing the leak the light stayed off for a while. It taught me not to assume the worst right away.

[Ryan4]

If you’re trying to decide what to do next, have you looked at the downstream sensor live readings? If those look reasonable and the code still shows, that’s when people start thinking about the converter, but there are a few other cheap culprits too.