S500 air suspension is fully up and wont go back down, what can i do

If you have seen lower back several times but the car raised by itself before, then you have to replace air spring. I replaced those with Arnott air spring.
If you had this problem for the first time, your air spring will be fine. If you have 4 corners air spring and the front one blows air in and out, your pump is okay.
Then I will first replace air suspension relay which costs $30. And you will have to check air hose connection. Finally you could try to replace air pump.

Hi. If the pressure can build up but doesn't hold when the car is off and doesn't leak air, check your air relive valve for debris or electrical problem. Try switch it with other side. If the problem go with switched valve - replace if not you may have an electrical problem with it.

first you need to disable turn electrical power off to the air suspension system then remove air spring solenoid valve electrical connector then remove air line to the air valve solenoid. you need have have raise vechicle safely support vechicle with suspension is fully down with no load.WARNING TURN OFF DISABLE ELECTRICAL POWER TO AIR SUSPENSION IF NOT UNEXPECTED INFLATION OF THE AIR SPRINGS COULD CAUSE SHIFTING OF THE VECHICLE WHICH IS DANGEROUS CAUSE INJURY TO YOU ALSO VECHICLE, DONT WORK ALONE IF YOU ARE WORKING UNDERNEATH VECHICLE HAVE FRIEND OR SOME ONE AROUND THAT RULE A PERSONAL MECHANICS OR ANY BODY WORKING ON VECHICLES.

Go into the trunk and see if the technician turn the switch back on if he did and the vehicle still wont level out turn the switch to off position, disconnect battery, wait full 60 seconds replace battery terminal, turn switch back to on then start car this should reset the air suspension.

Yes. the switch in the back that was unhooked is the adjustment method.

you mean air suspension. yes, there is a switch in the trunk i think on left side. turn it off , then open hood and on the air pump for suspension there is a regular air valve like on tires. Release the air from system, car will lower down.. then turn switch in trunk back on and start engine so the pump wont kill battery , and watch the car rise again to normal height.

may have bad level sensor on that side,and yes very very expensive to change suspension.

hummer h2 hydraulic keeps running depleting battery.

level change up to 20mm in 24 hours is normal.
if after start car returns to normal level you do not have problem yet.

all of the suspension is low or only one of the suspension?
if only one of the suspension very low... most the the problem is the air bellow in the suspension strut crack.. need to replace new suspension strut.