When a CAT converter goes bad, is the rotten egg smell a one time thing or constant from that time on. I had the rotten egg smell real bad about a week ago but now I don't smell it. The Cat converter is still on car and I have noticed that my idle races back and forth while sitting in park. Could my O2 sensor be blown too?

 

Your cat is probably shot. I'd replace the O2 sensor too.

 

I agree with my man on this one. Its seems straight forward.