Should I start with the oxygen sensor or catalytic converter replacement?

[Ryan19]

My 2016 Civic’s check engine light came on and the code reader says it’s the catalytic converter. The car seems to run fine, no loss of power or weird smells, but I’m worried this is the start of a very expensive repair. I’ve heard a failing oxygen sensor can sometimes throw the same code, so I’m not sure if I should just start by replacing the cat.

[TylerJW]

I had a similar thing last year. The CEL pointed to the catalytic converter, but the car ran fine. I started by swapping the upstream O2 sensor and watched live data. After that, the fuel trims steadied and the light stayed off for weeks.

[Nora_M]

I’d chill on replacing the cat yet. The code can come from a tired O2 sensor or a loose connection. I swapped an aging O2 sensor, drove for a bit, and the light didn’t come back right away.

[EllaL]

Another angle maybe the real problem isn’t the cat at all. I’ve seen folks chasing the cat while the issue was a small vacuum leak or a dirty MAF sensor that kept the readings goofy.

[Riley27]

Has anyone else seen this with no symptoms, and what did you end up doing first?