I recently installed bbk long tube headers and a catback Xpipe on my 03 Gt
I took it to a mechanic to get it checked out because my check engine light was on
He installed 2 new 02 sensors and wiring harness to reach the long tubes but the light came back on
My gas mileage has steadily dropped to about 12 MPG and im betting it has something to do with the light
Does anyone know how to fix this?

 

You need MIL eliminators or you need a tuner to turn off your rear o2's.

 

i heard MIL eliminators were just for off road xpipes not ones with cats

 

well if ur cats are hi-flow, your ecu may not be picking them up. so therefore, you may need some MIL's to correct that.. or, it could be something else, but i would try turning off the rear 02's before i started going crazy about what it could be

 

Go to your local auto parts store and get them to run a test on your CEL then come back here and post the code

 

Not sure why you posted in both sections but, FIND OUT what the code is first! It could be a lean code for all you know.