Hi guys, ive got a slight problem. Recently ive had the motor replaced in my 08 focus coupe s 2.0 duratec when I got it back it worked great and I took it up to colorado about 450 miles and on the way back it started sputtering and backfireing so I took it back to the shop becuase they still had warranty for their work on the car and I had a fouled spark plug and replaced it. I got the car back and it was still doing it just not as bad and only sputtes when cruising or accelerating slow but fine when you put more gas... Once again I took it back but this time they said they couldnt find the problem but my injectors and plugs were fine and they couldnt find a misfire. No codes were thrown untill today it gave me a 02 sensor bank 1 lean... Ive cleaned the maf, ran seafom through the intake, and added fuel injector cleaner through the car but it still slightly sputter and backfires. I run plus grade fuel, about 87 octane. Ive got a lightweight flywheel that they swapped to the new motor along with my clutch. Ive got 2 1/2 headers, highflow cat, k&n intake, 2 1/2 inch exhaust aero turbine muffler. All just little stuff but it hasnt had a tune yet. Any help would be appriciated if you need any more info just ask. Thanks in advance.