The O2 sensor will upset the overall fuelling system so deal with that before the actuator , the actuator /boost /low boost may just be a side result of the O2 and the ecu shutting down boost due to fuelling issues

Interesting thought. I was not taking the O2 sensor code that seriously because it does not appear with much freeze frame frequency. I was about to pull the trigger on a new sensor but checked the service records and an OE sensor was installed in March 2019, ~20k miles ago.

I reset all codes and drove ~60 miles today. The only fault that was present this evening was boost pressure control. Its runs pretty well so I am going to take my time and monitor codes for a bit before replacing anything.

The only exception is the PCV - I may order one of those given the old design indicates mine is pretty old.

The new design is only slightly different in the hose being altered as far as I know so just purchase the actual breather puk and save a lot of money.. you can buy the actual diaphragm and just replace that over here in the Uk.
There is a seal in the housing the breather slots into on the back of the head that can be changed as well , here below
http://www.peachparts.com/shopforum/diesel-discussion/399380-2011-gl350-om642-oil-consumption-solved.html

Today's goal was to remove the upstream O2 sensor and visually inspect. I was wanting to verify 1) I could get it off and 2) check for buildup before buying a replacement. I see the sensor from both the top and bottom, but no idea how to get a wrench in there. Any suggestions?

For the pre cat O2 you'll probably have to undo the band clamps (one below/behind the turbo and one underneath) and drop out the cat from below.

Unless you have one of those special O2 sensor sockets.

The exhaust on these is either stainless or titanium and the sensors just screw right out.

Just did this on my son's car (the 08 E320) with 230 k miles, had to replace the post O2 and temperature sensors, and it was easy peasy. Put the wrench on it, smacked the other end with the heel of my hand, unscrewed right out.

55 Nm torque if I remember correctly.

So if you can get an open end on the sensor, with it in place, give it a try, you might be able to get it out much easier than you think.

Thanks so much for all your help! I have not touched the turbo or exhaust yet so I want to be clear on which clamps you are talking about. I see the clamp downstream of the cat, just past the downstream o2 sensor. That looks easy.

Which upstream clamp would you recommend loosening? I see 2 options:
1. Loosen clamp in red circle + bolt in blue circle.
2. Loosen clamp by green arrow. No idea how to get to it, I can't even get my hands on the bolt.

Upstream O2 sensor is in yellow circle for anyone else that struggles. This is looking down from the rear passenger side of the engine bay.

Finally found instructions in WIS. I think I was previously confused about which part was the cat vs the DPF. When I searched for "catalytic" it came right up. So it recommends to go after the green arrow clamp to remove the cat. That may be a chore for another day!

I also found some diesel accumulating on top of the fuel filter. Man, this car is going to keep me busy!

The fuel filter leak is either the hoses where they connect onto filter, they get torn up inside when they get old and split but if you have the version with the water sensor then the o rings supplied in filter kits are often a loose fit even with the major brands of filters.
To this end purchase some 16mm id by 2.5mm cross section standard nitrile rubber o rings and use them, they are a good fit.

You can probably get the sensor from below quite easily if you have the correct type of socket to fit it, also have a look at inner bodywork and plastic arch liner as on my 2006 Jeep GC with same engine I can remove arch liner and have complete access to O2 sensor.

Thanks Murky! I am going to investigate the fuel filter another day. I think it does have the water sensor as I recall seeing some electronics on top.

Last update for today as I just put the car back together. If you remove the passenger side air filter, you can get at the O2 sensor from the top. I am going to buy one of those special sockets and see if I can get it off without dropping the cat. See attached pic.

Also, the CEL came back with a new code: "Check component B19/11 (Temperature sensor upstream of turbocharger). Plausibility". The same old boost pressure control code was present too.

On a positive note, I was able to suck a dead wasp from the headlight using a shop vac and a straw! So at least I crossed something off the ever growing list today! :D

Sorry wasn't online last night. Good that you have a copy of the WIS it is absolutely necessary.

One thing to watch out for, that nut holding the cat is a single use nut. You need to get a new one from the dealer if you end up pulling the cat.

General rule, any nut with a castle on top or any nut where they tell you a torque and a further angle is a single use fastener.

Update: Diesel leak on the fuel filter was fixed with a new o-ring and hose. Installed a new PCV as well as a preventative item.

As for codes, I was getting both the upstream O2 sensor and B19/11 temp sensor codes with increasing frequency, so I decided to replace the temp probe first. After that, the car ran for a bit with no CEL but just threw a light. Pulled codes and there were 2 instances of the same O2 sensor but apparently not enough to trigger a light.

What triggered the light was a first time code for me: 2624-001 Mass air flow sensor Sensor drift Offset drift under engine load The limit value of load point 1 has been exceeded.

After a bit of research I reset 2 adaptations with my Foxwell. I replaced some pretty dirty air filters ~5k miles ago and do not recall if I reset anything then. I will continue to investigate, but any suggestions from folks that have seen this before?

Y'all excuse while I go hug my 98 E300:).

Yes you have to reset adaptations on the OM642 after changing air filters.

The engine monitors the pressure differential on the drivers side filter with a pressure sensor in the top of the housing.

Another good to know, the factory interval is 40k/4 years on air filter replacement. I think that number was derived from testing in a class 100k clean room. Real world change them every other oil change. If you don't you will start really sucking oil down the crankcase breather.

I do the following routine on my vehicles:
Service A: oil change, basic inspection of underbody, brakes, suspension
Service B: oil change, closer inspections, air filter, fuel filter, cabin filter, brake fluid flush

722.9 transmissions
the first generation (in my sons W211 and W164) take the red fluid good for 15 years
the second generation (in my wife and my W212 and W166) take the blue fluid good for 5 years

PSA: WalMart.com has the Bosch 17236 upstream O2 sensor for the 07-09 OM642 on sale for <$40. I thought it might be a mistake, but I ordered and received one today that looks genuine, in a Bosch box. Super deal compared to most places selling it for $150+.

https://www.walmart.com/ip/Oxygen-Sensor/46065538

Another update: I reset all the adaptations and the mass airflow drift code has not come back. The temp sensor code has not returned since replacing that sensor. I finally got around to replacing the upstream O2 sensor yesterday - super each job with the right tool and several extensions. So far so good with the cheapo Bosch sensor from WalMart.com. Fingers crossed that the check engine light stays off for a while, but who am I kidding! Aldo did a brake fluid flush since the last documented service was over 100k ago.

Next up is a front lower ball joint. It is squealing like crazy, especially in cold weather.