RPMs are running high on a 09 370z

osvaldor

I installed Borla cats and full exhaust w a set of Stillen headers on my buddy's 09 370z. We had to order new O2 sensors for the headers since the fact ones were sealed, breaking 2 o2 sensor sockets borrowed from a local parts store (LOL). The o2 senors were not factory ones meaning we spliced the w the fact. connectors. Now the car is running at about 300-500 RPMs higher than normal, in addition to the service engine light being on and this bad boy is burning fuel (read 15 MPG driving very conservatively). I placed an additional washer on the header's o2 sensor and have o2 spacers for the cat backs.... What are we overlooking? Any and all help will be greatly appreciated.