Can someone tell me if a cat can go bad after 700 miles if the oxygen sensor is not replaced when it is bad.:banghead:

I doubt it...mainly depends on if the car is running rich without the O2 sensor.

Even running in open loop, there's no way a new cat would break down in 700 miles. Whoever told you that is feeding you garbage, go somewhere else...:thumbsup:

Thank you very much for your help