you should be good to go now. It was my understanding that the car has to run for a short time AFTER resolution of whatever the problem is before the check engine light turns off. It is my experience that O2 sensors should last 80-90k miles. They are a relatively easy replacement on most every car I have ever changed one on. 50k miles is probably stretching the life out of copper plugs but I have been known to run them up to around 70k on a Honda.