Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There has been a service bulletin for that type of concern. Here is what it states.
Date: March 31, 2009
Order No.: P-B-32.22/77
Except Vehicles Equipped with Active Body Control (ABC) (code 487a)
Vehicle Does Not Lower and/or "Airmatic Visit Workshop" Displayed in Instrument Cluster
If you receive customer reports in the above model vehicles of the Airmatic suspension not lowering and/or "Airmatic Visit Workshop" displayed within the instrument cluster, perform the following.
1. Remove right front underbody paneling
2. Unplug electrical connector from pressure relief valve (Figure 1, arrow).
3. Replace pressure relief valve. Carefully pull out the valve; the pressure piston is pressed out by a spring. It is important to observe the installation direction. To check whether the pressure relief valve is installed correctly, it must be possible to push the pressure piston down flush with the edge of the housing against the pressure of the spring.
4. Reassemble vehicle
5. Verify that the vehicle level rises and lowers.
Note: This condition is the result of the pressure relief valve (A9/1y1) not opening. The fault code C1517-032 [Correction times when emptying or filling are too long] may be stored.