Pre cats are located in the exhaust manifolds. They are for emissions on cold startups. The headers eliminate them, hence the need for mil's on odb-II cars.

Zach

2001 VW Golf GLS 1.8 Turbo
220hp/260tq
Porsche Cup 3s rims w/225 45 17 tires
Oettinger Grille, Votex Rear Spoiler, and way more mods