Hi,

I have a 2001 Honda Civic EX, 5sp, w/ 49 state emissions. Recently I was having idle issues (surging/stalling) when the engine was cold. At times I could literally floor the throttle and not receive any response from the engine. This would subside, however, if I let the engine warm up for five minutes or so.

I had no CEL and no DTC's. On a hunch, I replace the upstream O2 sensor. I replaced it with a Denso 234-4122. This solved the surging/stalling issue.

However, I now get a CEL w/ a code of P0420. If I clear the code out, it will stay off for about 30 miles, then come back on.

Any thoughts on why I'm getting this code after changing the sensor? Could it be the new sensor? (It was a different shape than the original sensor I pulled out.) Could it be the downstream sensor?

I know 99% of the time the 0420 is a bad catalytic converter. If you think it is the cat, please explain how the new sensor plays into it.

Thanks

 

I have always seen a 420 pulled, a cat replaced and a fixed problem. This problem is common, and might be under warrenty still.
As far as the new sensor being replaced and then getting the code. I think the new sensor is sensitive and finally read that there is a problem here.