I have a friend of mine who has a 2004 s60. He told me that his car had been making a funny rattling noise so he took it to a mechanic to check it out. What he was told was that he probably needed to replace his cat converter. Since then he has driven it and told me that it is loosing a little power but not severe enough to be the "limp" mode. I have put my code reader on it and didn't get anything back, But the CEL is on. He is pretty sure he wants to replace the converters.

My question is what are some things I should look for to determine if he needs to replace the cat converters or could it be as simple as an O2 sensor acting up? Would I need to replace both sensors or just the rear one? If I do end up replacing the converters can I use an aftermarket unit or do I need an OEM part. Ive heard some people say that using an aftermarked part will cause the CEL to come back on after a few hundred mile. There is a significant price difference between the two and if I only have to change the O2 sensors that would be even better.

Any and all input I get would be greatly appreciated.