320d E46 help needed

. i have just purchased a 2004 bmw e46 320d
A few problems
car cant rev over 4000rpm,
the dde light just came on after it was run on the diagnostics
turbo boosts but something feels like it is limiting it, today it idled fine and did not stall after the dde light came on.
it does not smoke, we were told to blank off the egr by pass valve and disconnectthe vacuum hose, we found this was already done. ie the return to the exhaust manifoldis blanket off. car is auto, it drives erratic, boosts then does not boost
it returned a error code: air pressure sensor, and injector 2 shows -5 when driving
we cleared the error code and then the turbo boosts but my car still can't rev past 3900 rpm. my questions are, what can be wrong? must i change the pressure sensor on the intake boost pipe? must i clean the maf sensor? must i change the boost vacuum controller ? the other night before the dde light came on, it made a strange alarm like buzz sound under the glove compartment area, -
the turbo is said to be in very sound condition.
any suggestions?