I'll assume that this vehicle is well beyond its warranty period. In that case, there is no need to take it to a dealer.
Ask around among people you know for a good, locally owned, independent shop. If the light comes back on take it there.
I'll assume that the code was P0420 and that the 0402 was a typo or something. In addition to checking O2 sensor operation one also needs to check for exhaust leaks. The P0420 does come from anomalous readings from the rear O2 sensor, so every other cause has to be eliminated before moving on to a converter replacement. If it does come to that you probably don't need to spend $2K - unless you're at a dealer.