Due to wear, high usage vehicles can experience ignition lock and tumbler failures. The ignition lock cylinder will generally require replacement to correct this concern.Google+
Car Problem Reports
Mercedes-Benz S420 High Usage Vehicles May Experience Ignition Lock and Tumbler Failures
Mercedes-Benz S420 Problem
Average Mileage: 175,333 mi (157,000 mi - 197,000 mi)
Not fixed yet.
Visitor, 1999 Mercedes-Benz S420, 197,000 mi
key ignition would stick at times/unable to turn key to start vehicle. Replaced ignition lock tumblers - solved the problem.