How My Mercedes-Benz W221 S-Class Nearly Betrayed Me on the Road Due to a Sensor Failure

One of the most common problems with the Mercedes-Benz W221 S-Class is the failure of the rear-level sensor. The level sensor rusts over time and breaks off, causing the car to lower to the ground or rise.

Background

It is my first Mercedes, and it’s been treating me well.  I ran into my first issue with it a couple of weeks ago.  I drove into work in the morning and had no issues; it was a normal drive. I drove home for lunch, and on my way down my driveway, the rear sounded and felt stiff.

I saw both sides of the rear were lowered to the top of the rear tires behind the wheel wells. Low rider!  The front was fine. I hooked up SDS and ran the quick test to see if it would throw a code.  It did not.  I also did not have the front dash warning about not driving because the car was too low.  Next, I ran the pneumatic tests.

The pump and front level control valves passed, but the rears both failed for the level control valves. I ran all the other tests, and they all passed, including pump and leak tests.  Playing around, I noticed you could manually go into each strut and add or bleed air.

To my surprise, I was able to pump the rear up. I re-ran the tests, and the rear passed this time, with a slight exception, which I only realized later.

When it runs the pneumatic tests, it attempts to level each air strut if needed.  Even when pumped up, the rear tests would stop at the test for a second and move on.  The fronts would take their time and feel it out.

I left it for 30 minutes, and it held its position.  Then I drove up the driveway and back down, a 30-second drive at best. When I returned to the garage, the rear was almost back to the floor.

What the heck? I hooked up SDS again and pumped up the rear. I left it overnight, and it didn’t move. I went up the driveway, and again, the rear end sank. I was puzzled. I did some reading, and everyone was pointing to the air struts.

It was “just one of the things you will have to do with airmatic suspension; they blow out.”  A local indy was looking at $1800-2000 to replace both rear struts and was convinced it was likely the issue.  I am not one to give in, though, and persist.

I was puzzled about both rear struts going out simultaneously and the ability to pump up the rear and then pass the tests via SDS.  It went down once I started driving, but I had never heard of a leak or conducted tests. Finally, I got a schematic of the airmatic system and started testing additional components.

I moved the car up and down the driveway again, checking the back wheels every 5-10 seconds. They were moving down. When I got back down the driveway, they were nearly down again. I hooked…