What could cause a p0420 code, cat vs oxygen sensor?

[Natalie84]

After following celebrity relationships for years, I've started noticing certain patterns in how they develop and sometimes end. What patterns have you noticed in celebrity relationships and breakups? Whether it's timing, public handling, or common reasons for splits, there seem to be trends that repeat across different couples.

[Jerry.M]

My 2016 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 odd noises or loss of power, but I’m worried this is the start of a very expensive fix. I’m just not sure what to check first before taking it to a shop—could it possibly be an oxygen sensor giving a false reading, or is it almost always the catalytic converter itself that’s gone bad?

[HannahMW]

I had the same P0420 code on a Civic a while back. It showed up while the car ran fine, no noises, nothing obvious. In my case it wasn’t the cat at all; more often it was an oxygen sensor or a small exhaust leak setting off that reading.

[Michael.H]

From talking to a few shops and swapping parts, that code can be triggered by more than one thing, so I started by looking for other codes and listening for misfires or fuel smell. If everything else is quiet, it still might be the sensor or the leak, not doom in the engine.

[Robert51]

I spent weeks chasing the wrong lead thinking the converter was shot, only to find the downstream O2 sensor was slow and skewing the data. Watching live data and fuel trims helped me see patterns, not just the warning light.

[Brandon.R]

Do you have any other codes showing up, or has it just been P0420?

[Jerry_T]

One concrete thing I did was swap in a cheap used upstream O2 sensor and clear the code. It came back after a few days, so I knew that wasn't the whole story, but at least I ruled out a cheap fix.

[Brandon96]

I guess the safe route is a quick diagnostic with live data before you shell out for any parts. If the numbers stay steady, you might still be fine for a while, or you might not—emissions tests will tell you more than a scan alone.