As a point of reference, my 98 E320 had its B service done at 28,245 miles. If the car was serviced at a Mercedes dealer, then the service booklet should show which service was performed along with the date and mileage. Once the service is performed, then the FSS should have been reset. If the dealer filled out the service booklet properly then it should also indicate which filters, fluids, and spark plugs have been replaced.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles