P0420 Low Efficiency Cat code - treatment options?

Hi guys,

I know this is a very common problem and I know there are several fixes discussed here. I have a specific question about possible fixes.

I've been told it's either the cat or the EGR valve. Problem is I my muffler store is telling me it's probably the EGR valve and my trusty shop mechanic is saying he thinks it's the cat. I don't know who to believe and don't want to replace something unnecessarily. Car has 192k on it and the cat & EGR appear original to the car.

I've got tremendous respect for automotive technicians but too often 'service' means just replacing parts they 'think' are the problem, only to discover it's not the answer.

Thanks in advance for any help,

-kj-