Simple.

It all has to do with CARB - a group of ineffectual bureaucrats with nothing better to do than make arbitrary laws that impede the free market and have little to no impact on emissions.

In CA [and I believe in NY, VT, CT & MD], any drivetrain part that is installed on a street-driven, pollution-controlled vehicle must either:

a) be 100% identical to the OEM part.

b) be "certified" by CARB and given an Exemption Order number.

If you're caught by a police officer (in the course of a vehicle search with non-certified parts installed (particularly eBay headers and such), you can be cited and ordered to go to the smog referee (in CA) for a thorough inspection / ass-reaming.

If you attempt to smog a vehicle with non-certified parts, you'll fail the visual inspection and never even perform the quantitative emissions test.

And the absurdity of it all is that there is no clear definition of this certification process. From what I've been able to discern, it is little more than a fee for parts like headers.

So yeah, it's a sore topic with me... I'm systemically allergic to bureaucracy.:a: