These fail all the time on 300Es. That is, all the 20-year old ones are now failing.

An independent Merc mechanic will replace it for you in 2-3 minutes, using special tools.

You have to get replacement cylinder from dealer, providing proof of ownership.

An independent might do this for you, providing a "loaner" cylinder until the correctly-keyed cylinder arrives.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles