Bad story. My father in-laws 98 S500 had the same carbon build up mentioned earlier. It started with the CEL, dealer thought it was O2 sens... nope, then some valve...nope, then air pump... nope.
It was the carbon in the airways. They pulled the heads, everything internal was worn from the heat build up. Heads to the MB machine shop + all new parts.
The head work alone is currently $7,000+
The dealer agreed to eat all the pre trial work + all parts on the head work, and 1/2 the labor.
This is a 65k mile car, and has a history of the same CEL issue.

Good news is if someone wants a 98 S500 Blue, with new heads, he will be shortly selling it for $25,000