check engine light turns on and off

it is P0421 the pcm has determined that the warm up catalyst efficiency for bank 1 is below threshold for the current engine operating conditions Thank you very much it is p0420 the catalyst that is beginign to fail i changed the o2 sensor and the light came back on after about an hour of driving and it is begining to smell like eggs.

My experience tells me that code p0421 is for bank 2 catalyst efficiency. Code p0420 would be for bank 1. Your Grand Prix does not have a bank 2 catalyst. Could your code been p0420? If so, I would suspect that you have a catalyst that is begining to fail. The longer you drive with this problem the more often the light will come on.