Is this on your S320? What year? Mileage? Any recent work? Condition of plugs? Wiring harness? (M104 harnesses in W140s?)
How's it running? Any changes?
OBD-I or OBD-II? Big difference.
The CE light on OBD-II cars can light for many, many things.
Have you had codes pulled? If not, you'll have to. If OBD-II, think about having an MB dealer or someone with Star Diagnosis System do it, as the tranny/AC/etc has codes that can be stored too. On 722.6 equipped cars, the tranny can light the CE light as well.
1998 C230 "Black Betty"