I have a 2003 K74 L200 which is serving me great except I had the occasional code 49 overboost problem on hard acceleration since I got it.
Recently I replaced the VGT actuator, solenoids and MAP sensor in the hope of solving this issue - in that order, because replacing just one part ended up throwing codes and causing new issues. I checked and found the new actuator moves a bit faster than the old one when the vacuum is released.

I am rid of the overboost problem and the car drives ok, but I have intermittent code 41 - "Main throttle solenoid valve system" and 52 - "Variable geometry control pressure system" codes.

There is also a muted thud whenever I release the throttle after the turbo has spooled up, which I don't know if is normal or not.

To me those codes are too general to aid in the troubleshooting, but I wonder if anyone here has an idea what is going wrong?

When you say you replaced "solenoids and Map sensor" which ones were they and were they Mitzi genuine or fleebay? Nik

I picked ones in the UK because the lead time is shorter, I don't really have a better source for parts, but they both look non genuine actually, with no mitsu stickers. VGT solenoid pair are part no MR577099 and the sensor was MR299300 brand JPL.
Are you saying these could be off from the get go?

Mmmm....I had similar prob......I got my twin solenoids (brown and black/) from milners....not ebay/amazon..... I took these solenoid valves out and tested them for air leaks on the work bench..One was leaking..There hasn't been any of those codes since...I struggled with overboost for a long time and eventually removed the turbo ,cleaned the vanes and reset it as per mitzy article on this site and its been fine since.
There are people much more knowledgeable than me on this site will chip in...Cheers Nik

@123hotchef , @Heathen @Alan G

Thanks Nik, I will try bench testing for leaks on the solenoid pair instead. I have gone through three pairs so I fear the problem is elsewhere. Especially since I have gotten rid of the overboost now and get 41/52 instead. Meanwhile I swapped back to the original MAP sensor with no change, same engine codes reoccur. I will look up the.Mitzy article.still hoping I don't need to pull the turbo but I will do that too if need be.

The sensor MR299300 isn't the MAP sensor, it is the turbo vacuum control sensor - they are often mis-sold as MAP sensors, because MAP sensors fail more often; the one you have is probably ok. An actual MAP sensor MR577031 might solve these problems - it attaches to the back of the intercooler frame. These errors can come up if the vacuum lines have been attached improperly - it is essential that the vacuum lines that feed the solenoids are attached to the correct nozzle of the solenoids.

Thanks @heathen, I have looked for docs on what vacuum line goes where and only found other posts here which describe things loosely. Pretty sure I got the right line to the right solenoid - since the valve actuate - but unclear on the nozzles. I will try to swap nozzles to see if that helps, and then look for MR577031 replacement.

I will also take a hard look at the turbo vacuum troubleshooting manuals here Diagnostic Codes and Troubleshooting for FBW Classics