Will the second O2 (the one by the cat) sensor effect how the car runs? I am under the impression that only the O2 sensor by the header will impact the air/fuel mixture. I am just checking before I put on a new downpipe

 

from what i hear, it just for for emissions..

 

the secondary o2 sensor is only for emissions. it will throw a CEL though if it is either not working or reporting that the cat is not working (or in place)

the primary o2 is the only one that affects if the engine runs correctly.