My 2002 Civic Si threw a MIL, P0420 (catalytic converter operating under
efficiency threshhold). I'm now very concerned because of the 80k/8 year
catalyst warranty of which my car is now at 79.4K miles!. I cleared the
code and it has not come back in ~100 miles. Now I intend to do the
diagnostic test in the FSM. It entails start the car, hold at 3000 rpm
until radiator fan comes on, then drive 30 seconds in 4th gear between 55-62
mph, and close the throttle for 1-2 seconds and repeat 4 times. I plan on
doing this procedure late tonight or tomorrow night when I can do this
somewhat peculiar maneuver. My concern is that it looks like it would be
easy to get a false negative. Does the step where I hold rpms at 3k trigger
some diagnostic mode or is it to ensure a properly warmed engine? Could I
do this on the way home from somewhere at operating temp or does the
ommision of the 3k step nullify the result? Should I bother sending a
dealer or Honda a letter stating what happened in case it comes back after
80k, will it help with a good faith repair? The procedure is supposed to
either trip the MIL again indicating a bad TWC or if not, to be considered
an intermittent failure where the code is cleared and life goes on. I am
hoping for the latter or a replacement before 80k, thanks!