sensors

I know that the upstream o2 sensor affects performance , but does the downstream sensor affect performance too? I know that the two sensors compare their data, but can a malfunctioning downstream sensor screw up fuel economy because it affects the (correctly functioning) upstream sensor?

hey guys. i got a turbo d17a2 and its been needing to go in for the emissions test for a while. now yesturday i went n i got a secondary o2 sensor and a defouler to clear the cel but ran into a problem i had no where to connect the 02 sensor. now theres a whole lot more background to this story...

So i want to build a new header and put it on my car but don't know what i should do about the 02 sensors???? (not putting a cat on cause of no need for inspection)don't want the engine light on,, what should i do and where should i adapt the o2 sensors into????