1999 Mitsubishi Galant 99 Mitsu Galant-P0421 Failed NYS Ins

Hello,

I've had this P0421 code for the last couple of years and have passed at least 2 emissions inspections. The light has been on for about a month before I turned it off. I just started experiencing the dreaded sulfur smell from time to time. I failed my inspection yesterday because I thought I could fool the computer by clearing the codes. Now, I have to wait for a drive cycle to complete. Since I am now getting the sulfur smell do I definitely need a new Catalytic converter? Or will a new O2 sensor do the trick. I have been quoted $255 for replacing the downstream O2. What should I expect from having to replace the Cat?