Code P0420 is "Catalyst system efficiency below threshold", which means nothing to me.
99 Accord, 4 cyl, auto, 115k, still runs good,check engine light has come on in past and clearing code and reseating gas cap would clear it for a long time. Now that won't work and code comes back on real quick even with a new gas cap.
I have a Haynes manual which is of no real help.
Where do I go from here, any help would certainly be appreciated.
Thanks in advance.

 

you probably need a new catalytic converter

 

Test the 2 O2 sensors before replacing the cat. At that low mileage, the car must not have been running right to clog the cat. You said resetting the ECU clear it for a long time, how long before it came on again?

 

Code P0420 pops up when the ECU doesn't detect a large enough variation between the primary (upstream) and secondary (downstream) O2 sensors. In other words your cat is no longer working efficiently, "cleaning" the exhaust enough.

In my experience, at this mileage and age it's not uncommon for the cat to need replacing. And typically with no other codes or issues present, replacing the O2 sensors will not fix the problem. (I will say though that if you believe you have the original factory primary O2 replacing that with OE or OE equivalent [Denso] should be part of your next tune up and could improve your fuel mileage.)

Something you can try before spending money on new parts is to do a little test/trick with the secondary O2. Buy two spark plug de-foulers/anti-foulers at the auto parts store. I've always seen them in packs of two, but you'll need two of the longer ones either way. Drill out the end of one so that your O2 sensor threads in completely. Then stack the two de-foulers between the exhaust bung and the sensor.

like this: [exhaust]{normal de-foul}{drilled de-foul}[O2 sensor>----

reset the ECU and drive around for a few days to a week. If the code stays away the cat needs replacement. Also this will not pass emissions inspection and technically is illegal. Just so you know.

 

I got a 99 Corolla, the code pop up out of no where. I reset it once which was about a few months ago already and it never came back. How long before the code comes back if it is indeed the Cat? I was just wondering so that I can prepare what might be costing me.
Thanks.

 

not to sound patronizing but this is a Honda-Tech Accord forum, I'm not sure if the same solution applies or not.
In general I think within two driving cycles (start up, drive to reach full opp. temp, cool down) if the code is not back it's considered "intermittent" and everything is fine for the moment.

 

No worries. I worked on my 90 Accord and this Corolla, their EGR systems are pretty much similar.