I have a 2000 accord v6 3.0L. Has around 200k miles. Car runs great, but get horrible gas mileage. Almost 12mpg. And I drive the car easy. The code p0420 is a catalytic converter code. So I just replaced the cat. The old cat "looked" fine. And the O2 sensor looked OK also. Changed the spark plugs as well with NGK.

Clear the code and drive the car and about 30 miles later the check again light comes back on. Same code. P0420. What is going on?

Anyone have any ideas?

 

O2 sensors can be bad without "looking" bad. If you can get a code reader and look at the data while the car is running, the 1st o2 sensor voltage should swing wildly from .1v to .9 if I recall correctly, while the second sensor should stay more or less stable around .6v-.8v. When the computer is seeing wild fluctuations on the second (downstream) sensor, it assumes the catalytic converter isn't doing its job and throws a p0420. So the issue could be the cat, or the sensor. It seems as if in your case, it is the sensor. I always replace the sensor first in these cases since it's usually cheaper.

Someone correct me if I'm off on the voltages, this is just from memory.

 

Make sure your not carbon tracking through the Wire Boots, replace o2 sensors ideally every 80k, replace your air filter, fix exhaust leaks,.scope fuel pump etc. Obd2 you monitor Data and it should tell you what you need to know. If you don't know how to do this. Find a good Diag shop. A lot of the time the first thing they suggest is replacing the Cat w/out even really diagnosing it. I'v seen misfires and improper combustion cause this code as well.