The brief
The owner kept getting a low-oil warning on the B200's dash even though every dipstick check showed the engine sitting at the upper mark. He'd stopped trusting either the warning or the dipstick, which is fair enough, and brought it in for a definitive answer.
Modern engines have an oil level sensor in the bottom of the sump that feeds a level reading to the dash. It's handy when it works. But when the sensor itself fails, it can report a low level that isn't real, so you get a warning light on a full engine. The first job is to work out which one is telling the truth, the dipstick or the sensor.
The diagnosis
On the scanner, the live data confirmed the suspicion: the oil level sensor was sending a low reading that didn't match the actual quantity in the sump. We cross-checked it properly, a dipstick check, a visual look at the sump, and a drain-and-measure of the oil, and all three agreed the oil was right at spec.
So the engine was fine. The sensor was lying. That's a part you replace, not something you adjust, and once it's out the dash goes back to telling the truth.
The work
Enough oil was drained to drop the sensor cleanly out of the bottom of the sump, the failed unit came off, and a new Mercedes-spec sensor went in with a fresh seal. The engine was refilled to the correct level, and the stored fault codes were cleared.
Then a road test, watching the dash to confirm the level reading stayed correct and the warning stayed off.
The outcome
The warning light is off, the level reads correctly on the dash, and nothing came back on the road test.
The B200 went home with the dashboard telling the truth again. A false low-oil warning is the kind of fault that's easy to either panic over or ignore, and neither is right, so getting the sensor checked against an actual measurement, and replacing it once it was proven faulty, is the way to put it to bed.