There is not really a service F, it means a time interval when you need the get the service A or B.
Here is the explanation from the MB manual:
Maintenance System: FSS (Flexible Service System)
Model Years: MY1998 - MY2004
Models: 129, 140, 163, 170, 202, 203, 208, 209, 210, 215, 220, 230, 463
With the FSS maintenance system there is a minor or major service designated as "A" or "B" respectively. If the last service performed was an A-service, than the next service will be a B-service. For subsequent services; if the distance driven since the last B-service is less (<) than 14,000 miles, than an A-service should be performed. Consequently; if the distance driven since the last B-service is greater or equal (=) than 14,000 miles, than another B-service should be performed. Service intervals are based on oil quality, which is determined by the vehicle's oil quality sensor.
MY1998 - MY1999 the instrument cluster service reminder displays one wrench. Any additional work to the respective A or B service scope is calculated using the maintenance sheet.
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