What should i check first for p0420 on a civic: o2 sensors or cat converter?

[John91]

My 2017 Civic’s check engine light came on yesterday and the code reader says it’s P0420 for catalyst efficiency below threshold. The car seems to run fine, no weird noises or loss of power, but I’m worried this is the start of a very expensive repair. I’m just not sure if I should start by checking the oxygen sensors or if the catalytic converter itself is already failing.

[RileyG]

I had that same code on my Civic once and it felt perfectly fine too. In real life, that P0420 code often comes from sensor readings, not a dead cat. The downstream O2 sensor drifting out of spec is a classic trap that makes the system think the catalyst isn’t doing its job.

[Kevin71]

On a different car I just swapped an upstream O2 sensor and the light stayed off for months after. No catastrophic cat drama, just a sensor that had learned a bad habit.

[ScarlettM]

Another time I chased this and found a tiny exhaust leak near a clamp could throw off the readings enough to trigger it. It wasn’t the converter, just a leak making it look weak.

[PatrickT]

Could it be worth pulling some live data and seeing how the sensors actually behave before assuming the worst?