A corner of the car that visibly sits lower than the others after sitting overnight, a bouncy or unsettled ride that wasn't there before, or a dashboard warning light referencing the suspension system all point to the single most consistently discussed mechanical weak point across the luxury SUV and sedan segment.
Air suspension replaces conventional coil or leaf springs with air-filled bladders whose pressure is actively managed by an onboard compressor, giving genuinely excellent ride comfort and, on some vehicles, adjustable ride height for off-road capability. The trade-off is that air springs, like any rubber component under constant pressure cycling, degrade and develop slow leaks with age — and once a spring starts leaking faster than the compressor can keep up, that corner of the car sags. The compressor itself is also a wear item, and height sensors (which tell the system how far each corner has sagged) can fail independently and trigger warning lights even when the springs themselves are fine.
This is near-universal enough on the vehicles below that independent specialists generally treat air suspension condition as one of the first things to check on any used example — right alongside, not instead of, whatever the engine's own known issues happen to be.
This symptom shows up across several unrelated engines and drivetrains — each has its own root cause and its own full writeup.
Observe the vehicle at rest after it's been sitting for several hours or overnight — a corner sitting visibly lower than the others is the clearest sign of a spring leak.
Cycle the suspension through its height settings where accessible, and listen for the compressor running excessively or struggling to hold height.
Ask whether any component (springs, compressor, sensors) has already been replaced, and get documentation — a partial repair doesn't guarantee the remaining original components are sound.