I have had my 2012 E350 for the past 50k miles (just hit 120,000 miles), runs great always doing standard maintenance myself, but had a CEL pop up with all 6 errors for the P0155 bank 2 sensor 1 O2 sensor four days ago. I ordered the upstream sensor from FCP Euro and it should be here this week. I was also due for my scheduled A(1) service so I thought I would look at the O2 sensor to see about how I will swap it yesterday. I found pictures online of the W210 sensors where the engine is removed, but haven't found any threads on the W212 M276. Also on page 431/7118 of the service manual its very vague about any procedure on how to get to the clips. I'm not a big dude and can barley get my hand between the catalytic converter and the back top side of the engine. Using my cell phone I was able to snap a picture of the green O2 sensor plug. From the top of the engine its tucked back under a ledge on the fire wall so there isn't any access there. Any tips on how to disconnect this without the sarcasm of pulling the engine? When the new one comes in I'll look at the clip but that area is super tight, I know on my wife's 08 MDX I had to put a strap on the exhaust and drop it a bit two swap one of those O2 sensors out.

 

Update: Switched out the O2 sensor, I'll posts pictures later of everything. It was easier to do than I thought, but a super tight confined area. To start off I think it was San Francisco that killed my O2 sensor; when I unplugged it about a tablespoon of oil fell out of the pig tail. Over the last couple years I would get a random minor smell or whiff of burning oil (maybe a handful of times a year or so for couple minutes), but never loose oil or could see any leaking oil in/around my engine. After reading some forums there are the two rubber cam plugs in the back of the valve covers that need to be replaced from time to time but I have yet to replace them. In San Francisco recently I parked over night on a very steep hill facing up. The following day the CEL popped up with the O2 PO155 errors. I also noticed a slight rough idle when at red lights for the next few days. I can only think that parking on the steep hill that some of the oil collected at the back of the covers leaking out and dripped perfectly down onto the O2 harness running down into the coupler shorting out the sensor.

To change the sensor I had to pick up a 3 pack of 24" long screw drivers from Napa since I have never had a use for any. Without these changing the Bank 2 Sensor 1 would have been impossible. The only access to get to the coupler clip is from the top of the engine next to the fire wall. With a flash light between the engine and the fire wall I used the screw drivers to push the clip and the standard to pop it open. I was super cautious though because on my wife's Acura the connectors were really tight. Once I un-clipped the connector I pushed it almost all the way apart and then used the screw driver from another angle to see how tough it was top clip back on. It was really smooth and easy to clip back together.

After popping it free again I used the screw driver to pop it out of the metal clips that hold the sensor pig tail in place. Crawling under the car and reaching up I pulled the connector out when the oil poured down right next to my head. I was barley able to clean the opposite side of the connector with shop towels since the area is so tight. After removing the old sensor and putting the new one on, I was able to barley fit the connectors back together. Once I aligned them up and slid them together about 1/8" or 1/4" I sat the connectors on the metal holding clips. Going back to the top side of the engine I used the standard screw driver to press the O2 Sensor side clip into the wire harness holder. Once it clicked in the two connectors were still about 1/4" together; at that time I was able to use the screw driver from another angle and press the connectors together hearing the clicking of the clip. It was much easier to do than I thought especially with the 24" screw drivers.

If this would have been Bank 1 Sensor 1 there is a lot of room to get your hands up in that area and you can remove the air filter housing on the M276, but being on the driver side Bank 2 Sensor 1 its tight with the steering column. The best access from underneath was having your head face the back of the car around the transmission area and basically bear hugging the engine block. I did have to remove the last wire clip before the connector because it would tear up your wrist. I have a few pictures to post for better details. After doing the upstream sensor on the driver side I would say changing the front breaks take way longer than swapping this sensor just to give an idea on time.

 

Great job -- both on the work and the write up

Those O2 sensors are really buried in our cars.