The brief
The S300L had been sitting noticeably lower on one corner overnight, the ride had grown harsh over expansion joints, and the car was slow to come up to driving height after the doors were closed. Three signs, all pointing at the air suspension.
AIRMATIC is the S-Class's air suspension, an air strut at each corner kept inflated by an onboard compressor, with the ride height controlled electronically. When an air strut develops a slow leak, it deflates over time, so the car sits low and lopsided in the morning, and a deflating bag rides harshly because the air spring can't do its job. That's the picture here, and it's the common age failure on these struts.
The diagnosis
The scanner was logging a slow-leak fault on the affected corner. With the car raised, soapy water on the strut produced bubbles where the air bag joins the can, the classic failure point, confirming it.
The other corners checked clean, but with the front struts being the same age and the same exposure, doing them as a pair was the sensible scope rather than chasing the second one back here in a few months. There's no re-sealing a worn AIRMATIC bag, it gets a new strut.
The work
The AIRMATIC pressure was discharged on the front, the old struts came off, and a new Bilstein pair went on, the air lines and harnesses reconnected.
Then the system was re-pressurised and the AIRMATIC calibration routine run on STAR, so the suspension controller re-learned the correct ride height all round on the new parts.
A settle and a road test confirmed the car sat level, rose at the right rate after parking, and rode the way an S-Class should.
The outcome
All four corners sit level, the ride is back to S-Class smoothness, there's no overnight sag, and the car rises at the normal rate after it's parked.
The S300L went home riding the way it was meant to. Doing the front struts as a pair means the air suspension is sorted properly, not patched one leak at a time as the next bag gives up, and on a car this size that refinement is the whole point.