Did they tell you why the check engine light came on (e.g., what trouble code(s) it was sending)? Have you checked out Jim Forgione's excellent web site and read about the dreaded dtc 19? As the 119's get older, they fall out of the narrow tolerance that was initially set by the factory. The dealers' solution is much more expensive than his. My 400E had a dtc 19 and the dealer wanted to sell me a new computer for $3,000 about ten years ago.

Check out http://www.k6jrf.com/MB_S500.html under Diagnostic Trouble Code 19 (scroll about 1/3 down the page).

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles