I bought some used cats in great condition, like 40k miles, for my G37 coupe for like $600 and they looked pretty much identical... but when I cross matched the part numbers they are actually different... The used cats I bought came off a 2013 G37s coupe 6mt and I have a 2009 G37s coupe 6mt. I wasn't going to drop $2000 on new ****ing cats (I don't have original ones so I can't return the cores) so this was a great deal and I wasn't thinking all the way through it. B08B2-1NC0A / B08B3-1NC0A are cats for my 2009 G37 and B08B2-1ET0B / B08B3-1ET0B are for the 2013 G37.. man I don't wanna do all that work taking the old cats off and the other ones don't even fit.. or do they? Does anyone know the difference between them?

 

I believe the flanges may be different.i know they changed at a certain year model but dont now the exact year. Flanges on exhaust manifold side.

 

If the problem is only the flanges (hopefully at one end), and the dimensions are otherwise identical, then a skilled welder could cut & replace the flanges. Granted, that's more money, but still not this two grand number you're avoiding.

 

The flanges for a 2009 and 2012 g37 coupe are the same. The dimensions will be the same also. The difference might be with internal material. All years 370z and G37 cats and test pipes physicaly fit each other.

 

I dont forsee this being a problem. Part number wise I'd be stuck on a powersteering pump only to find out the differences between the pump for my car and the one for a Titan was a bracket.

It's common info that 2010 then 2013 were revisions, so technically those cats should perform better than 09 versions. I would install them either way and find out.
Either way 40k mile 2013 cats will always be parts that will never depreciate so money wise you wont loose.