First of all have them tell you why(read codes) the check engine lamp is on. Second the engine wiring harness might be covered under "GOODWILL" waranty if the car has less than 100,000mi.
MB used terrible wire that the insulation FALLS off, this is a design problem! Of course the replacements have been redesigned.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator