I had the same problem w/ 01 Civic EX, CEL taken after 2nd long trip. I left the light on for a week when i was going to purchase the o2 Simulator from "Diablo Motorsports". The day I was going to purchase the sim the light went away. I have the HP 4-1 Race w/hi flo cat. So this could possibly work out the same w/ your civ. Try running it a week with the light on, see if it goes away...