Greetings All,
I want to share my recent experience with my 2013 GL450 and it randomly sinking suspension. My issue began about 2 months ago when i began noticing the right rear sinking slightly overnight, but would always pump up quickly on a restart and might go several days or more before I once again noticed it lower.. Since the vehicle has 70K miles on it, this was much earlier than I expected to encounter issues with the airmatic but began researching online reading everything I could find on the forums. In the early stages, I never had any warnings or error codes.when scanned.. Of course the issue progressed to more frequent drops at first always in the right rear and then eventually both sides in the rear., finally after several weeks it displayed a warning message in the instrument cluster with the down arrow after a trip to the grocery store. It was a slow and very bumpy ride home over what I thought was smooth pavement! So, I pulled out my scanner (AUTEL) and read the codes. I had a C155664 compressor air sensor for system pressure has a malfunction - Implausible signal, and C156C00, the pressure line system is leaking.. Some information, but not really a great lead. I have a leak. Well, yes, the suspension goes down so it must have a leak, but where is the leak? I started at the right rear and checked the airbag with soapy water but did not detect any sort of leak. I place a jack stand under the control arm, started the car, it pumped up and stayed up for a couple of days, it hadn't done that in a quite a while. Why did it stay up now when it was leaking down in an hour or less before i pulled it into the shop? So, moving on, I pulled the front right side fender liner to get to the airmatic pressure control block and checked all connections there for leaks, none were found at any external hose connection. The pump rarely ran, and when it did seemed to work completely as expected. After some more reading online, I decided that my issue was most likely the airmatic valve control block so ordered a replacement. I installed the new valve block and started the vehicle. Everything pumped up and looked great. I let it sit in the shop and within the next hour or less it fell lower on ALL 4 WHEELS now! it had never done that, only on the rear, what's up with that? It was late one afternoon, so I left it and went in to do some more research online and reading. I found a ton of information on airmatic but virtually all is/was for the earlier versions 2012 and prior, but I began to slowly read those posts anyway. After having searched for my codes, I also began searching and reading by the text description of my error. I noticed, earlier versions of airmatic used the same description but different codes. Now that;s clever for whatever reason.

So the next day, I go out to the shop, start the GL and it pumps up. I clear codes and take it for a short drive, about 1 mile and all looks fine, I park it and let it sit overnight, everything stays up. OK, let;s venture out a bit farther, I take it 2.5 miles to the grocery store. All good, I go in get a few things, return only to find the rear axle now low on both sides all within 15 minutes, I start it up and get the message instantly on the dash, telling me I have a low rear end. Yup, confirmed!! I drive it home again.and back into the shop. Could my new valve block be defective?? I didn't buy it from the dealer, but did get a reputable name in the air shock business. So, more reading and research, surely someone has fixed this problem. I am beginning to suspect the issue lies with the airmatic pump or a leak in a line somewhere, but why the random sinking versus always one side. Finally, I see a new clue on a post for the earlier version of the pump that say the solenoid for the air release valve sometimes sticks and causes an undetectable loss of air pressure. Well, definitely worth a look, or I would order a new pump. So, I pull my pump out to give a through check. I pull off the cover with the two torx screws as described and my 2013 has a totally different configuration that appears to be a pressure transducer, not a solenoid as in the earlier versions, yet the two pumps appear to be pretty similar from outward view. I look much more closely at the transducer and consider it probably performs the same function, just using newer technology. I notice on the tip of the transducer that it slips into a smaller cavity in the pump which appears to be the pressure chamber of the pump and the tip is surrounded by a small o-ring, with the outside of the transducer chamber sealed by a larger o-ring. Upon examining the center o-ring closely, I notice it seems to be slightly flattened and a wee bit loose in the body of the pump. What the heck, I will replace it just for grins, it can't hurt. So, I pull out my o-ring supply box and find the best match I had it is a 4.3mmX2.4mm a tiny little bugger that is pretty thick. I slipped it in place on the transducer and put the transducer back into the pump. Reinstalled the pump, fender cover and wheel, then lowered the vehicle, to normal height on the lift, wheels just touchng the floor, and started it up. The pump ran some, but no messages or codes. I then lowered the lift completely and let it run. I turned it off and let it sit for an hour and it never dropped. After about 3 hours, I needed to use the vehicle for an errand and took it. I monitored system pressure throughout the trip with my scanner and noticed it pumped up to 13-15 bars and then would drop to .5 bars. almost instantly, yet the vehicle drove perfectly. It appears to be the nature of the beast. Anyway after several days now, I have to say it has been working perfectly. My leak appears to have disappeared for the price of a 25 cent o-ring. OF course a new air pump would have fixed it as well for several hundred dollars. I probably didn't need the .airmatic control block either, so now have a used spare in my parts collection. I now have a much better understanding of how the system works. We'll call the cost of the control block my education expense not to mention the hours pouring over search results trying to figure out what was going on. or where the problem could be. Sure, throwing parts at it would eventually haven't gotten it fixed.for some cost. Few things cannot be fixed with sufficient application of money, on cars anyway. I hope this description helps other who come after me. It is a fairly easy fix actually and one could do it with the pump installed in less than an hour if they wanted to. This might not be your issue, you could have leaking bags, or a broken line or any number of things, but this is one item that is worth checking and only takes a few minutes.