A few weeks ago I started my 2002 S60 and it sounded horrible. Very rough and barely moved. Turns out I had to replace the ignition coils, but ever since then I have had the P0420 CEL code stay on. Is my Catalytic converter toast? I haven't smelled the sulfur smell that is associated with a bad catalytic converter. Is it more likely a seal problem or a O2 Sensor?

 

Have you had the code read? Usually P0420 code is a secondary air delivery problem. I would check for a cracked vacuum line, loose gas cap, etc. I really don't think the problems are related.