Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Actually- before condemning the Catalytic Converter- there is a Technical Service Bulletin that states the problem may NOT be the converter, but a software correction for the PCM (on-board computer). The P0141 is for the internal heater for the rear oxygen sensor. The PCM uses this heater to get the sensor to normal operating temps within a matter of seconds on cold starts. Before replacing the converter, I would suggest replacing the rear oxygen sensor, and having the Dealer check your PCM to make sure it has the correct software version installed. Clear the codes and road test the vehicle.
Here's the Honda TSB:
SOURCE:Honda Service News July 2004
TITLE:S/M Fix: DTC P0420 Troubleshooting
APPLIES TO:2003-04 Accord, 2003-04 Element
SERVICE TIP:The troubleshooting procedure for DTC P0420 (catalyst system efficiency below threshold) in the 2003-04 Accord S/M and the 2003-04 Element S/M is wrong. It was written before we had the current software problem and the updated software to correct this DTC. Make this change in your copies of the appropriate S/M:
Don't replace the catalytic converter unless the software has already been updated and DTC P0420 sets again; doing so wouldn't provide a lasting repair and could result in a warranty debit.