Just got it a month ago knowing it had a few issues but was up for the task. What its doing is shuttering under full throttle. It had a boost leak up at the intake. Fixed that and the boost did increase but it still shutters. Previous owner told me it has ko4s in it and it does have exhaust but no other mods. Boost climbs to around 18 psi with no problem and holds it. Just has the shutter and it doesn't even start that until around 4500 rpm or so. Ran the codes and the only ones coming up all point to o2 sensor. Bank 1 sensor 2. Code p1118, p1114 and p0140. Do i have waste gate trouble? Or is it trying to shut down do to over boost? The last run I took it on it did go into what I'm guessing it's limp mode and would only boost about 8 psi. Can anyone give me any pointers on what to look for. This is my first s4 and still learning it. I had an a4 with mods but this seems like a different animal.