I have changed the exspensive o2 sencor and engine light went on for two months then went out. i think bulb burt out but went to autozone and the code po420 came up. do i just need to change that sencor which is 160 bucks or do i need to change the cat

The code means 'Catalyst System Efficiency Below Threshold (Bank 1)' which means the computer thinks your catalytic convertor is not functioning properly. However, since the convertor itself doesn't have any sensors, how the computer knows this is via the O2 sensors (both before and aft of the cat). However, if the O2 sensor is dying or faulty it could give erroneous readings that would trigger this code, and the most common one to fail is the downstream, or Bank 1, sensor. The cat is very expensive to replace and usually that would be the last thing you would want to try. O2 sensor failure is quite common but you said you already changed it so.
Another reason you might see this code is if the internal coolant temp sensor is faulty or not functioning. If the computer thinks the coolant is warmer than it really is, it will run the fans more and keep the engine cooler. A cooler engine produces worse emissions than a hotter one so the cat wouldn't get up to the temperature it needs to be to function properly, thus triggering the code. Again, not an expensive sensor to replace and every dealer should be able to test them with a volt meter before blindly replacing them.

Finally, your original cat (and pre-cat) are warrantied by federal law for 8 yrs/80K miles so if your '02 is under 80K it should have been covered by Acura