Should I replace the catalytic converter or diagnose first?

[AlexanderNL]

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 odd smells or loss of power, which has me wondering if it could be something simpler like an oxygen sensor giving a false reading. I’m just not sure if I should start by replacing the expensive part or dig deeper with more diagnostics first.

[IsabellaNR]

I've seen that exact scenario on a Civic. The code pointed to the cat, but it wasn’t always the catalytic converter. In my case it was an upstream oxygen sensor. I swapped that first, cleared the code, and then drove around for a while; the light stayed off and there was no noticeable change in performance.

[EvelynQW]

If you can read live data, watch the downstream O2 sensor and the fuel trims. A slow or erratic downstream reading or big swings in LTFT/STFT often points to a sensor issue or a leak rather than a dead cat.

[JohnXM]

Do you have access to a scanner that shows live sensor data, or only the basic code reader? If you’re limited to codes, you may be guessing more than fixing.

[Edward.R]

Sometimes the warning is spurious and comes back after a few days even after you fix the obvious stuff. I once waited a few thousand miles to see if the light would return, and it did, which forced me to dig deeper instead of replacing parts at random.