2001 Nissan Maxima SES Light is on - Error code P0420

Hi,

I have a Nissan Maxima 01 which has done 138k miles. Recently when I was driving in the interstate, the Service Engine Soon light came on. I took it to a service station and got it checked. It seems that the downstream O2 sensor has failed and needs to be replaced. I cannot get this problem fixed for the next few days and I was wondering if this would affect the engine performance, if I were to change this after a week. Aslo I read inb many forums that the SES light comes back on after a few weeks of it being replaced. Is it a problem with just the O2 sensor or the whole cat? Kindly advise