I have a stock 2002 Gs300 and the car recently came up with a p0420 and P0430 code. Stating its a bad cat and bad o2 downstream. Im thinking of replacing the cat with an aftermarket header and i found some OBX headers on ebay but its a 1 piece header. Im not sure what is the following piece that i am missing to make it complete. can someone tell me what else im missing? Please see the following picture of the unit.

 

That is a turbo manifold, not headers.

 

I agree, that is a turbo exhaust manifold. they should sell non-turbo exhaust manifold, that either bolt right up or need slight modification.
p.s. dont forget exhaust gaskets

 

They told me that it should fit a stock US GS300 but i wasnt sure.

 

that small two bolt flange is for an external wastegate, and four bolt flange bolts the turbo to the manifold.

the exhaust manifold you need should look similar to picture below
check out www.rockauto.com for decent prices, usually they have the lowest prices around if trying to stay stock(ish).

 

I have thought about giving one of these a shot, but I decided to cheat the test instead.

https://www.summitracing.com/parts/f...stionsanswers/

 

Ouch. Suddenly my GS400 with two headers, two pre-cats, four O2 sensors, one resonator, and two mufflers, seems cheap. Yet we just get hit repeatedly for less....

 

We see that you are located in California, which means you would need a CARB-compliant replacement cat and header. Fortunately, we offer one of these for your GS300 that is a direct bolt-on installation and includes all necessary hardware/gaskets. You can find some more info about it here on our website:
https://www.magnaflow.com/products?partNumber=452843

If you have any questions about it, let us know!

 

Hey Magnaflow, when are you going to update your catalogue to reflect that the SC430 and GS430 do NOT use the same Y-pipe, #49085? This part number ONLY fits the SC430, it's several inches too short to bolt into a GS430 without modification.