What could trigger a P0420 code if the catalytic converter seems fine?

[Noah.H]

My 2016 Civic’s check engine light came on, and the code reader says it’s P0420 for catalyst system efficiency below threshold. The car runs fine, no odd sounds or loss of power, and it just passed its last emissions test a few months ago. I’m wondering if this could be triggered by something simpler than a failed catalytic converter itself, like an exhaust leak upstream or a faulty oxygen sensor giving a bad reading.

[Ronald48]

Yeah P0420 can be one of those annoyances that isn’t the catalytic converter failing outright. A tiny exhaust leak after the cat or a sluggish downstream oxygen sensor can fool the system into thinking the cat isn’t doing its job. If you have live data, peek at the bank 1 sensor 2 readings and see if it’s slow to switch or staying high when the car is warmed up. A loose clamp or small leak around the exhaust flange is easy to miss but common and can trigger that code.

[Eric35]

I had a Civic with the same code. I kept chasing the cat for weeks, but the real culprit was a drifting downstream O2 sensor and a corroded heat shield rattle. Replaced the sensor and the code cleared, car felt the same. It’s easy to jump to the cat, but the sensor is cheaper and more likely to drift first.

[NicholasG]

My experience was that a small exhaust leak around the gasket at the mid-pipe caused P0420 to show up after the car heated up. No power loss, emissions test passed last year, but the leak drifted readings enough to set the code. I fixed the leak, reset the code, and drove for a few weeks before it popped again, maybe due to something else. It’s messy.

[Mia27]

Have you checked the downstream O2 sensor readings or any audible exhaust leaks? If you haven't, that might be the first thing to verify.