I'll usually rely on previous threads for answers to my questions. I know the 0304 knock sensor code is sometimes a ghost code when there are other things that may be wrong with the car. But I've had the knock sensor replaced about 5K miles ago. I just checked my ECU after the intial problem was fixed, but now it only gives 0304. Should I reset it again and hopfully it will be gone?

reset it and check it when it comes back

You O2 sensor is probably getting "tired". My ECU will trip a RH O2 sensor code on long trips (greater than 400 miles). But a week or two of driving around the city and the light will go away.