The service engine light came on a friends '99 E300D yesterday. The car runs fine but he wants to know what it means and what to expect as to what might have failed. Any ideas or is just another electronic crap shoot.

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Mark
82 300SD 110k
91 Caprice SS
92 Jetta TD
97 Cadillac Concours(300hp)
84 Celebrity 4.3L diesel