Background story...I've had a P0420 CEL on my 03 RSX for well over a year now. Always eventually comes back even after resetting it. Back in November 2010, I was actually able to pass smog here in Cali after clearing the CEL, running it through a few drive cycles and smogged it before the light could come back on.

Finally decided to take care of this since I'm sick of looking at the CEL, plus bad gas mileage. Question is, have any of you Cali folks ever just had a CARB legal universal cat welded on at a muffler shop instead of taking a chance with another used OEM cat or buying a new OEM cat?

Also, could this possibly be a non-cat related problem? Maybe one or both of the o2 sensors? I've never thrown a CEL related to the o2 sensors, just this P0420.

 

i had the same problem before, it could be your 02 sensor or the cat
ive changed my 02 but the CEL came back on after a few months, so now im running a defouler and as of now its still good

 

i'm in the same boat. my 0420 has been on for about 8 months. dont have to smog til later this year (i'm in socal)
i kept resetting it through my ultragauge.. and usually doesn't come on until about a week later every time.

you can try the defouler, which is probably what i'm going to end up doing. could be the o2 sensor.
worst comes to worst, you'll have to get a new cat. i've seen some used ones go for pretty cheap (not sure how reliable they are though)

 

Thanks for the responses. I think I'll switch out the secondary o2 sensor first since that's pretty cheap and take it from there.

 

If you are getting this CEL and seeing bad gas mileage replacing your secondary O2 sensor might not be the solution. The primary controls all of your fuel consumption. It is the bread and butter of the air/fuel mixture. The secondary O2 sensors main purpose is to check the catalytic converter and make sure it's flowing properly and etc, it should not effect gas mileage.
But it's throwing me off cause usually the P0420 codes mean the cat is bad or the secondary o2 sensor is bad.

 

So if bad gas mileage isn't occuring, could it be that the secondary o2 sensor could be at fault instead of the cat?