The problem is most likely one of the O2 sensors in the car. If you take the car to a dealer or a shop they can hook the car up to diagnostic equipment that will be able to tell exactly which sensor has gone bad by measuring the voltage going to each sensor. This will cost some money, but It may be cheaper than just guessing and replacing all of the sensors and hoping you fixed it.

If you decide to replace the O2 sensor yourself, and are correct in replacing the one that was bad, your check engine light will shut off automatically and the computer will clear the codes itself after the PCM is able to successfully run and complete the emissions tests that caused the code to be thrown in the first place. This may take a day or two depending on how much driving you are going to do. In order for the car to run the emissions tests, certain criteria have to be met first i.e. car started and shut down a certain number of times, engine maintain a certain RPM for a designated amount of time.

Worst case scenario is that the catalytic converter is bad. In most cases these devices will outlast the car, but they do fail occasionally. GM vehicles especially have a bad rep for this happening. I own a 2001 Trans am and just went through this process and needed a new cat. Due to the high volume of incidents with these cars, GM issued a bulletin extending the warranty of this part. $700 part and I got it for free under warranty.

Finally, to finish out your question. If you don't have the time to get this looked at right away there is no harm that will be done to your car during your trip. You may see slightly decreased gas mileage, but aside from that your car is basically just "polluting" a little more than usual.

Hope this helps!

po420 means check engine. Most of the time it's due to a faulty sensor. That is a long drive I would get it checked out just in case. Breaking down in the middle of no where is never fun, I am speaking from experience.

Have a safe trip :)

wiki answers had the following

Code P0420 is a code for Catalyst efficiency. The computer is seeing the post Catalytic converter o2 sensor having too much activity. The Cat. should be cleaning the exaust of Hc, co, and Nox. The post cat o2 will read the o2 content in the exhaust and if it moves up and down too much or to high and low, the computer interperates that as the cat not ridding the exhaust of it's pollutants. This indicates that the converter is unable to preform properly. The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail. Many other engine operating system failures can cause the converter to not work properly

i would change the o2 sensors before and after there cheaper then a new cat and something if you have a jack some jack stands and a few tools you can do your self. then go to an auto parts store and ask if they can clear your check engine codes or disconnect your battery for 10 mins will clear them some times. if it comes back you need your cat replaced and that will cost some more money. it should be ok for the trip might help it with the highway miles to get it warm and it might clean it up a little if its plugged. my gf's mom needed a cat for a 96 jagg and it was around 1200 to have it done.

First of all DONT GUESS is the cat colder on the downstream side or hot ? if its cold put on a cat if not look at the o2 sensords . if the cat breakes up your going to clog the exaust and get some hillbilley replace it all for whatever he wants to charge you ??????????? good luck Michael