Code 420 after doing tune-up
 

1998 Accord, replaced plugs, wires, dist cap and rotor. Now getting an occasional Code 420. I clear it with hand held scanner and it comes back every now and then. It doesn't come back immediately after I clear it.

Anyone have any ideas?

That code is for the catalytic converter. It could be the converter, a bad gasket, a dirty connection where the O2 sensor in the converter plugs in.

How's your exhaust system? How clean is the outside of your 2nd O2 sensor?

O2 sensors actually measure DIFFERENCE in oxygen between outside & inside. If an exhaust leak is blowing exhaust onto the outside of the sensor, or if it's got oil dripping on it, or something similar, that'll throw off the measurement. Then the ECU will throw code 420 because the sensor signal looks wrong.

Code 420
 

If there are no other codes you will soon need to replace your Cat converter. P0420 means there is a difference in the efficiency of Bank1 and 2 Oxygen sensors. 98 Accord V6 has a TSB on this issue. Sometimes the trouble is intermittent but I am almost certain that it will get progressively worse with time. Mine did.

Replace the Catalytic Converter.

I bought mine on eBay from mufexpress for approx $125 including shipping. Theirs came with the bolts and gaskets. 2 months and no CEL. Perviously it would come back within a day or two of resetting the code. FRT is half hour for this job.

If you go to the stealership they will want to replace both O2 sensor and Cat conveter and will cost you more than $1000.

I have this same problem... Can clear code, and within the a week or so of driving it will come back... Cat has been replaced, O2 sensors have been checked, and are good... ?

Corvetteking, The place I bought my cat converter from http://stores.ebay.com/Muffler-Express-Distribution guarantees theirs to keep the check engine light off, if installed properly. I had also priced a Walker Cat Converter at a local auto parts warehouse. They recommended buying their Walker Cat with two matching O2 sensors. I went with mufexpress because they only deal with exhaust systems.

If you have no other codes and if you replaced yours with an after market, and there are no leaks, and O2 sensors are working and clean, its possible that the new cat does not have enough substrate to handle this car's computer needs.

Knock on wood, my light has not come back.

It is a magnaflow high flow cat... It was not a cheap converter, or no name brand...
The There are no leaks, no other codes, and the O2 sensors are perfect...It was purchased from the local dyno shop here, and put on there as well...