1) I know there are 4 O2 sensors on our cars. Are the 2 front O2s located right behind the turbos? These are the ones that are hard to replace, corret? The 2 in the rear, are they before the cats or after the cats?

2) In case of piggie pipes, which 2 sensors are you re-locating? The 2 sensors right after the turbo or the 2 sensors in the back?

Thank you for your help.

1) yes/yes, esp. the driver's side one/after the precats

2) relocating the ones after the precats to be after the main cats