I have 2009 Honda Pilot EX-L. Whenever I fill up the tank, the code P0420 shows up. When fuel tank is bellow half, the code disappears. I have been able to duplicate this scenario multiple time. And no, it is not the fuel cap. However, when i drive long distance, 200 to 300 miles, the code does not show up on full tank. It is only when I drive around town with full tank, it show up. Any ideas? I have more than 180K miles on the clock. The spark plug, air filter, etc are new.