Did you check if there is 12 V going to the pump? There is a white plug on the pass. side of the trunk. Take your spare tire out and remove the plastic cover so that you can get to the plug with a DVM.

There are a couple more people who have a problem similar to yours on the board. None have solved it yet but for at least one guy the ride height is maintained, he just cant go to the high or olow setting.

I would first check voltage and open the pumpkin to seeif the compressor is operational.

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.