What is a "rear" or "secondary" catalytic converter for?

[Xist]

So, the second cat may be the equivalent of a 1.44 MB floppy?

[California98Civic]

Originally Posted by jcp123

It could be California. I know most automakers have done away with California-specific cars and just put CARB-approved stuff on for all USDM cars, at least for the most part....

Hyundai was still doing a separate CaliSpec as recently as the 2011 Elantra, according to https://www.hyundaipartworld.com/.

Elantra 2011 with CAT/Manifold:


Elantra 2011 with CaliSpec second CAT (#4):


Elantra 2011 with FedSpec midpipe (no second CAT):