My 2002 Civic Si threw a MIL, P0420 (catalytic converter operating under\nefficiency threshhold). I'm now very concerned because of the 80k/8 year\ncatalyst warranty of which my car is now at 79.4K miles!. I cleared the\ncode and it has not come back in ~100 miles. Now I intend to do the\ndiagnostic test in the FSM. It entails start the car, hold at 3000 rpm\nuntil radiator fan comes on, then drive 30 seconds in 4th gear between 55-62\nmph, and close the throttle for 1-2 seconds and repeat 4 times. I plan on\ndoing this procedure late tonight or tomorrow night when I can do this\nsomewhat peculiar maneuver. My concern is that it looks like it would be\neasy to get a false negative. Does the step where I hold rpms at 3k trigger\nsome diagnostic mode or is it to ensure a properly warmed engine? Could I\ndo this on the way home from somewhere at operating temp or does the\nommision of the 3k step nullify the result? Should I bother sending a\ndealer or Honda a letter stating what happened in case it comes back after\n80k, will it help with a good faith repair? The procedure is supposed to\neither trip the MIL again indicating a bad TWC or if not, to be considered\nan intermittent failure where the code is cleared and life goes on. I am\nhoping for the latter or a replacement before 80k, thanks!