About 6 months ago the "Check Engine Light" in my Civic came on for
the first time. I reset the ECU (by momentarily pulling the
Backup/Hazard fuse) and the light stayed out for at least a couple of
months. Since then however, the light has come back on a couple of
times (with many weeks and miles in between). After the latest
incident, I had the ECU code read at Autozone. This is the only info
that was available:

"P0420 Catalyst System Efficiency Below Threshold (Bank 1)"

Here are some clues: There has been no noticeable driveability or gas
mileage degradation. Also, whenever the light has illuminated, it has
always occurred a minute or two after exiting a highway and traveling
slowly (20-30mph) on secondary roads. The car has about 165K miles.

Is it probable that the car has a slowly failing catalytic converter or
could one of the oxygen sensors possibly cause this error code as well?
Though I'd like to drive the car another year or so, I'm certainly
not inclined to replace the converter ($$). Is my risk from not making
any repair at this point limited to possibly not passing an emissions
test?

Any comments or suggestions would be appreciated.