ALWAYS replace the first sensor identified by the diagnostic system before replacing other sensors or the CAT.
Replace the after-CAT sensor now, and reset the code -- go to Autozone and buy the sensor, and they will reset the code (turning the light off) for you, or they will let you use the code reader and reset the codes yourself. Then, wait for a few days to see if the light comes on again, then use the code reader again to see what the fault is.
I assume you read the codes, then reset them, then the Check light came on again. This code (P0420) is one that might come on, then the conditions that caused it go away on their own but the light remains on for years. I doubt you are in this situation, but the proof would be to reset the computer (using the scan tool) and then if the Check light remains out, then you have no worries. If the light comes on again, AND the new code is also P0420, then you need the after-CAT sensor.