[AF4iK]

I had a similar issue with the drive train malfunction warning and lack of boost that was getting progressively worse.
The check engine light was on due to a lean condition in bank 1 and I had the following stored codes:

118001 - Mixture control: Mixture too lean
120308 - Boost pressure control, plausibility: pressure too low
195013 - Differential pressure sensor, tank ventiliation valve, working range: absolute pressure too low
119304 - Rail pressure sensor, plausibility: pressure too high before engine start
102001 - Air mass, plausibility: air mass too high

Initially, I only had the lean mixture code but over time the others cropped up.

In my case, I did 3 things which mostly solved the problem.

1. Charge Pipe
I Smoke tested the system and discovered a couple of major leaks in the charge pipe coming from the two connecting pipes (see photo). Not sure what each of these does but the smaller connection completely broke off while I was trying to disconnect it. The larger connection (MAP sensor?) had a failed O-ring and was also leaking from around the outside seam of the pipe-join.

I removed the charge pipe from the vehicle and reattached the smaller connection with epoxy. I then sealed up all the leaks as best I could with RTV.
Oh I forgot to mention that the end of the charge pipe that goes into the turbo completely broke off too. I ended up rebuilding the end using some ABS piping, a rubber coupler and a couple hose clamps from Home Depot. Ghetto but it works for now.

Once I reinstalled the charge pipe I smoke tested again. There was still a tiny leak coming through but I couldn't pinpoint it. It was much better than before so I decided to leave it as I have a new charge pipe on the way.

I went for a test drive and found that light acceleration from a stop felt slightly better. The CEL (after being cleared) did not return during the test drive.
However, I was still getting almost zero boost and the drivetrain malfunction was still present.

2. Exhaust Flap
Using Bimmerlink for Android, I opened the exhaust flap (previously it was on "AUTO"). This immediately made a huge difference. I was now producing about 16.5 psi of boost at WOT. Take a look at the attached screenshots of before and after.

[UPDATE] Just discovered that the vacuum line to the exhaust flap wasn't even attached lol. Not sure how the heck making a software change to "Open" made such a big difference?

3. Wastegate Actuator
Next I decided to check the wastegate actuator. I have a pneumatic actuator which should be calibrated to 5.9 inHg in order to close the wastegate flapper.
Using a handheld vacuum pump, I measured the current setting at 9.5 inHg! I removed the actuator, adjusted the rod to shorten it by 6 turns which brought it back to spec (approx).
Went for a test drive again and this time was getting about 18.6 psi. This is over the 17 psi of rated boost but I left it like that.
My brake pedal and steering wheel also felt a little firmer.

After all of this, the stored codes disappeared except for the lean condition which comes back after a while (also triggering the CEL). I'm hoping that the replacement charge pipe will take care of that once and for all (should be here in a few days). Also, my fuel economy improved from about 13.4 to 10.5 L/100km (~17.4 to 22.4 MPG).