Sounds like bad O2 sensor dude. Just went through this with my headers. My one plug was slightly cracked so the wires werent making a complete connection on the one sensor. I was getting 10-12 city and 15-17 highway....pulled the plug and now I'm at 18 city and 28 highway. Plug for you is easy to replace anyone 2011 and up will be a bitch. They use different plugs and the dealerships can't even order them...having one specially made :/
__________________
| 445whp398tq|
|BW S259 Turbo|3" Downpipe\Exhaust|ID1000|AeroMotive|Tial|EcuTek|BC BR|S0-4|MaffFab|
MKGs-Single-Turbo
|