Why would p0420 show up and could the o2 sensor be the culprit?

[Paisley60]

My 2017 sedan’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 smells or loss of power, but I’m worried this is the start of a costly exhaust system repair. I’ve heard it could just be a bad oxygen sensor, but I’m not sure how to tell without just throwing parts at it.

[Madison_R]

I had P0420 pop on my old sedan. I started with the cheap stuff first and swapped the downstream oxygen sensor. It was only a couple hundred dollars all in, and the light stayed off for several months after that.

[Jason_L]

Sounds like a sensor problem more than a dead cat if the car runs fine. I’d want to see live data—fuel trim numbers, downstream sensor reading—before guessing. Do you have a scanner that shows that stuff?

[JacobLJ]

I'm not sure I believed it at first, but I did a quick live data check and watched the downstream sensor switching slowly. I cleared nothing, and the code came back after a week. Could be the sensor or something else.

[KyleT]

One time I checked for exhaust leaks around the manifold, ended up chasing a loose heat shield for days. It didn't fix the code, which made me wonder if I was barking up the wrong tree, but I never got a solid answer.