first of all: does the rear of the car sag? does it drive differently? if you have a problem with the pneumatics of it, you'll know very soon.
the compressor is in the back of the car, under the floor. you might have to remove a cover or two to get at it but it's inside, not underneath. the sensors are adjacent to the struts in the rear wheel wells.

also worth considering: have you installed any aftermarket electric parts that might interfere? i used to get a suspension fault message (and other random faults) on an E39 because i put cheap LEDs into the halos. even swapping them left to right gave me completely different errors, all of which disappeared when i put the original bulbs back in.

there are a fair few threads on air suspension DIY, you might want to check those.

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